This document shows an example plugin that will extend Inbound Forms to be able to add a lead to a custom taxonomy on form submission. It also shows how to create a custom field type that provides a dropdown of available taxonomy terms.

inbound-customization.php

<?php
/*
Plugin Name: Inbound Now - Customizations (Do not disable)
Plugin URI: http://www.inboundnow.com/
Description: Helps add leads to custom taxonomies
Author: Hudson Atwell
Version: 1.0.0
Author URI: http://www.twitter.com/atwellpub
Text Domain: inbound-pro
Domain Path: /lang/
*/


class Inbound_Customizations {

    /**
    * Initialize Class
    */
    public function __construct() {
        self::add_hooks();
    }


    /**
    * Load hooks and filters
    */
    public static function add_hooks() {
        /* add taxonomies */
        add_action('init', array( __CLASS__ , 'add_taxonomies' ), 10 );

        /* extend form settings  */
        add_filter('inboundnow_forms_settings' , array( __CLASS__  , 'extend_form_settings' ) );

	    	/* Add handler to process unit type field type */
	    	add_filter( 'inbound_form_custom_field' , array( __CLASS__ , 'inbound_form_custom_field' ), 10, 3 );

        /* Add form processing hander */
        add_action('inboundnow_form_submit_actions' , array( __CLASS__ , 'process_forms' ) , 10 , 2 );

        /* Filter by taxonomy */
        add_filter('parse_query', array( __CLASS__ , 'filter_leads' ) );


    }


    /**
     * Add taxonomy
     *
    */

    function add_taxonomies() {
        self::register_agents();
        self::register_sources();
        self::register_status();
        self::register_type();
        self::register_unit_type();

    }

	/**
	*  Register agents taxonomy
	*/
	public static function register_agents() {
		/* Agents */
        register_taxonomy('wplead_list_agents','wp-lead', array(
    		'hierarchical' => true,
    		'label' => __( 'Agents' , 'cta' ),
    		'singular_label' => __( 'Agents' , 'cta' ),
    		'show_ui' => true,
    		'show_in_nav_menus'	=> false,
    		'query_var' => true,
    		"rewrite" => true
    	));
	}

	/**
	*  Register sources taxonomy
	*/
	public static function register_sources() {
        register_taxonomy('wplead_list_source','wp-lead', array(
    		'hierarchical' => true,
    		'label' => __( 'Sources' , 'cta' ),
    		'singular_label' => __( 'Source' , 'cta' ),
    		'show_ui' => true,
    		'show_in_nav_menus'	=> false,
    		'query_var' => true,
    		"rewrite" => true
    	));
	}

	/**
	*  Register status taxonomy
	*/
	public static function register_status() {
        register_taxonomy('wplead_list_status','wp-lead', array(
    		'hierarchical' => true,
    		'label' => __( 'Statuses' , 'cta' ),
    		'singular_label' => __( 'Status' , 'cta' ),
    		'show_ui' => true,
    		'show_in_nav_menus'	=> false,
    		'query_var' => true,
    		"rewrite" => true
    	));
	}

	/**
	*  Register type taxonomy
	*/
	public static function register_type() {
        register_taxonomy('wplead_list_types','wp-lead', array(
    		'hierarchical' => true,
    		'label' => __( 'Types' , 'cta' ),
    		'singular_label' => __( 'Type' , 'cta' ),
    		'show_ui' => true,
    		'show_in_nav_menus'	=> false,
    		'query_var' => true,
    		"rewrite" => true
    	));
	}
	/**
	*  Register status taxonomy
	*/
	public static function register_unit_type() {
        register_taxonomy('wplead_list_unit_types','wp-lead', array(
    		'hierarchical' => true,
    		'label' => __( 'Unit Types' , 'cta' ),
    		'singular_label' => __( 'Unit Type' , 'cta' ),
    		'show_ui' => true,
    		'show_in_nav_menus'	=> false,
    		'query_var' => true,
    		"rewrite" => true
    	));
	}

  /**
   * Extend Form Settings
   *
   */
  function extend_form_settings( $settings ) {

	   $settings['forms']['options']['add_to_agents'] =  array(
    		'name' => __('Add to Agent', 'leads'),
    		'desc' => __('Assign lead to agent', 'leads'),
    		'type' => 'select',
    		'options' => self::get_agents(),
    		'class' => 'main-form-settings',
    	);


        $settings['forms']['options']['add_to_source'] =  array(
    		'name' => __('Define Source', 'leads'),
    		'desc' => __('Assign source to lead', 'leads'),
    		'type' => 'select',
    		'options' => self::get_sources(),
    		'class' => 'main-form-settings',
    	);


        $settings['forms']['options']['add_to_status'] =  array(
    		'name' => __('Set Initial Status', 'leads'),
    		'desc' => __('Set initial status of lead', 'leads'),
    		'type' => 'select',
    		'options' => self::get_statuses(),
    		'class' => 'main-form-settings',
    	);

		  /* Adding taxonomy dropdown field type */
  		$settings['forms']['child']['options']['field_type']['options']['dropdown_unit_type'] = __("Dropdown - Unit Type Taxonomy", "inbound-pro");

    	return $settings;

    }

	/**
	*  Listens for attachment uploader field type and renders it
	*/
	public static function inbound_form_custom_field( $form, $field, $form_id) {

		/* only render field if 'attachments' type is selected. */
		if ( !$field || $field['type'] != 'dropdown_unit_type'){
			return;
		}

		/* get attachment settings */
		$args = array( 'orderby'=>'asc' , 'hide_empty'=> false );
		$unit_types = get_terms( array('wplead_list_unit_types'), $args);

		$options = array();
		foreach($unit_types as $type){
			$options[$type->term_id] = $type->name;
		}


		$form .= '<select name="unit-type" class="custom-dropdown" '.$field['required'].'>';
        $form .= '<option value="" disabled selected>Please select a unit type</option>';


		foreach ($options as $key => $value) {

			$form .= '<option value="'. $key .'">'. $value .'</option>';

		}
		$form .= '</select>';


		return $form;
	}




    /**
     * Get Agents
    */
    function get_agents() {

        $args = array( 'hide_empty' => false );
        $terms = get_terms( 'wplead_list_agents' , $args );

        $array = array();
		$array[''] = 'Please select option';
        foreach ($terms as $term) {
             $array[$term->term_id] = $term->name;

        }
        return $array;
    }

    /**
     * Get Sourcess
    */
    function get_sources() {

        $args = array( 'hide_empty' => false );
        $terms = get_terms( 'wplead_list_source' , $args );

        $array = array();
		$array[''] = 'Please select option';
        foreach ($terms as $term) {
             $array[$term->term_id] = $term->name;

        }
        return $array;
    }

    /**
     * Get Statuses
    */
    function get_statuses() {

        $args = array( 'hide_empty' => false );
        $terms = get_terms( 'wplead_list_status' , $args );

        $array = array();
		$array[''] = 'Please select option';
        foreach ($terms as $term) {
             $array[$term->term_id] = $term->name;

        }
        return $array;
    }

    /**
     * Get Types
    */
    function get_types() {

        $args = array( 'hide_empty' => false );
        $terms = get_terms( 'wplead_list_types' , $args );

        $array = array();
		$array[''] = 'Please select option';
        foreach ($terms as $term) {
             $array[$term->term_id] = $term->name;

        }
        return $array;
    }

    /**
     * Get Statuses
    */
    function get_unit_types() {

        $args = array( 'hide_empty' => false );
        $terms = get_terms( 'wplead_list_unit_types' , $args );

        $array = array();
		$array[''] = 'Please select option';
        foreach ($terms as $term) {
             $array[$term->term_id] = $term->name;

        }
        return $array;
    }

    /**
     * Process Form
     *
     */
    function process_forms ( $form_post_data , $form_meta_data ) {

        $form_settings = $form_meta_data['inbound_form_values'][0];

    	parse_str($form_settings, $form_settings);

		/* get lead by email */
        $lead_id = Inbound_API::leads_get_id_from_email( $form_post_data['wpleads_email_address'] );

        /* assign lead to agent */
        if (isset($form_settings['inbound_shortcode_add_to_agents'])) {

           /* Add agent to lead */
           wp_set_post_terms( $lead_id , $form_settings['inbound_shortcode_add_to_agents'] , 'wplead_list_agents' , true );

        }


        /* assign lead to Source */
        if (isset($form_settings['inbound_shortcode_add_to_source'])) {

           /* Add agent to lead */
           wp_set_post_terms( $lead_id , $form_settings['inbound_shortcode_add_to_source'] , 'wplead_list_source' , true );

        }

        /* assign lead to Status */
        if (isset($form_settings['inbound_shortcode_add_to_status'])) {

           /* Add agent to lead */
           wp_set_post_terms( $lead_id , $form_settings['inbound_shortcode_add_to_status'] , 'wplead_list_status' , true );

        }


        /* assign lead to Unit Type - legacy method
        if (isset($form_settings['inbound_shortcode_add_to_unit_type'])) {
           wp_set_post_terms( $lead_id , $form_settings['inbound_shortcode_add_to_unit_type'] , 'wplead_list_unit_types' , true );

        }
        */

        /* determin unit type if applicable */
        print_r($form_post_data);exit;
        if (isset($form_post_data['unit-type'])) {

           /* Add agent to unit type */
           wp_set_post_terms( $lead_id , $form_post_data['unit-type'] , 'wplead_list_unit_types' , true );

        }
        //print_r($form_post_data);exit;

    }

    /**
     * Filters leads by custom taxonomy
     */
    public static function filter_leads( $query ) {
    	global $pagenow;

        $qv    = &$query->query_vars;
    	if ( $pagenow != 'edit.php' || !isset($qv['post_type']) || $qv['post_type'] != 'wp-lead' ) {
	    	return;
    	}

    	$taxonomy  = array('wplead_list_agents','wplead_list_source','wplead_list_status','wplead_list_types','wplead_list_unit_types'); // change to your taxonomy


	    foreach ($taxonomy as $tax) {

    	    if ( isset($_GET[ $tax ])  && $_GET[ $tax ] ) {
	            $term = get_term_by('id',$qv[$tax],$tax);
		        $qv[$tax] = $term->slug;
        	}
	    }


    }


}

new Inbound_Customizations;