eventON Custom Field Functionality

eventon-custom-field.php

<?php
/*
Plugin Name: eventON single custom field
Plugin URI: https://eventon.com
Description: A text field for eventON submit event form
Author: Basilis Kanonidis
Version: 2.0
License: GPLv2 or later
 */

/*
 * Config the name
 */
define('EVENTON_SINGLE_FIELD_NAME', 'price');

/*
 * Prepare the Class
 */

class CustomEventOnSingleTextField
{

    public function __construct()
    {
        add_filter('evoau_form_fields', array($this, 'evoau_custom_fields_to_form'), 10, 1);
        if (!is_admin()) {
            add_action('evoau_frontform_evo'.EVENTON_SINGLE_FIELD_NAME, array($this, 'evoau_custom_fields'), 10, 6);
        }
        add_action('evoau_save_formfields', array($this, 'evoau_custom_save_values'), 10, 3);
        add_shortcode('event-custom-field-value', array($this, 'get_event_custom_field_value'));
    }

    public function evoau_custom_fields_to_form($array)
    {
        $array['evo'.EVENTON_SINGLE_FIELD_NAME] = array(ucfirst(EVENTON_SINGLE_FIELD_NAME), 'evo' . EVENTON_SINGLE_FIELD_NAME, 'evo' . EVENTON_SINGLE_FIELD_NAME, 'custom', '');
        return $array;
    }

    public function evoau_custom_fields($field, $event_id, $default_val, $EPMV, $opt2, $lang)
    {
        echo "<div class='row evoau_table'><p>";
        $evoau_value = ($EPMV && !empty($EPMV['evoau_' . EVENTON_SINGLE_FIELD_NAME . '_value']) && $EPMV['evoau_' . EVENTON_SINGLE_FIELD_NAME . '_value'][0]) ? $EPMV['evoau_' . EVENTON_SINGLE_FIELD_NAME . '_value'][0] : '';
        echo '<p class="label"><label>' . ucfirst(EVENTON_SINGLE_FIELD_NAME) . '</label></p>';
        echo '<input type="text" id="evoau_' . EVENTON_SINGLE_FIELD_NAME . '_value" value="' . $evoau_value . '" name="evoau_' . EVENTON_SINGLE_FIELD_NAME . '_value" class="fullWidth" style="width:50%;height:19%">';

        echo "</p></div>";
    }

    public function evoau_custom_save_values($field, $fn, $created_event_id)
    {
        if ($field == 'evo' . EVENTON_SINGLE_FIELD_NAME) {
            // for each above fields
            foreach (array(
                'evoau_' . EVENTON_SINGLE_FIELD_NAME . '_value',
            ) as $field) {
                if (!empty($_POST[$field])) {
                    add_post_meta($created_event_id, $field, $_POST[$field]);
                }

            }
        }
    }
    public static function get_eventon_custom_field_value($eventId='')
    {
        if($eventId == '') {
            $eventId = get_the_ID();
        } 
        
        $a = get_post_meta($eventId, 'evoau_' . EVENTON_SINGLE_FIELD_NAME . '_value',true);
        if ($a) {
            return $a;
        } else {
            return false;
        }

    }

    public function get_event_custom_field_value($atts)
    {
       extract($atts);
       if(isset($id) && $id ) {
            $eventId = $id;
       } else {
            $eventId = get_the_ID();
       }
        
        $a = get_post_meta($eventId, 'evoau_' . EVENTON_SINGLE_FIELD_NAME . '_value',true);
        if ($a) {
            return $a;
        } else {
            return false;
        }        
    }

}

new CustomEventOnSingleTextField();

if (!function_exists('get_eventon_custom_field_value')) {
    function get_evenon_custom_field_value($eventId='')
    {
        echo CustomEventOnSingleTextField::get_eventon_custom_field_value($eventId='');
    }
}

I think this is what I'm looking for, to allow an attendee to add in a custom message to their ticket purchase - but, I'm not sure how to implement the code. Should this be installed as a plugin? If so, how do I hook it into EventOn so that it's available as an option?

For example, here is an event where I would like to have a text box that a customer could add a custom phrase for their art project:
https://paintedpeonydecor.com/events/farmhouse-centerpiece-box-copy/

Thanks for your help!