Quick and dirty method I wrote up to integrate a Ninja Forms into Mailster, make it add subscribers to the main Mailster subscribers list. (WordPress)

WordPress_Ninja_forms_mailster_integration.php

<?php
// http://developer.ninjaforms.com/codex/custom-form-action/
// https://kb.mailster.co/mailster-for-developers/

add_action('init', 'ninjaforms_mailster_integration_init');

function ninjaforms_mailster_integration_init() {
    if (!function_exists( 'mailster' )) return;

    add_action( 'ninja_forms_mailster_integration', 'action_ninja_forms_mailster_integration' );
}

function action_ninja_forms_mailster_integration( $form_data ) {
    if (empty($form_data['fields_by_key'])) {
        return;
    }

    $fields_to_collect = array(
        'firstname' => 'nome_1503181849365',
        'lastname' => 'sobrenome_1503181864093',
        'email' => 'e-mail_1503181872211',
        'phone' => 'telefone_1503181882828',
    );

    // define to overwrite existing users
    $overwrite = true;

    // add with double opt in
    $double_opt_in = true;


    $fields_needed = [];
    $userdata = [];

    foreach ($fields_to_collect as $type => $fieldname) {
        if (empty($form_data['fields_by_key'][$fieldname]['value'])) {
            continue;
        }
        $userdata[$type] = trim($form_data['fields_by_key'][$fieldname]['value']);
    }

    // Add to mailster now
    $userdata['status'] = $double_opt_in ? 0 : 1; //1 = subscribed (default) , 0 = pending, 2 = unsubscribed, 3 = hardbounced

    // add a new subscriber and $overwrite it if exists
    $subscriber_id = mailster( 'subscribers' )->add( $userdata, $overwrite );

}

Make sure you map fields correctly to the ones you have in your form.
See array on line 18.