Populate GiveWP v3 form dropdown from URL value

populate_v3_dropdown_from_url.php

<?php
add_action('givewp_donation_form_schema', function($form) {
	//Get field by name attribute
    $field = $form->getNodeByName('field_name_here');
    // if the field is not found, return
    if (!$field) {
        return;
    }

    // get the current default value
    $defaultValue = $field->getDefaultValue();

    // if the value is set in the query string, use that as the default
    // eg. yoursite.com/?value=somevalue
    if (isset($_GET['value'])) {
        $defaultValue = give_clean($_GET['value']);
    // if the value is set in the referrer query string, the form is being embed so we should use that instead
    } elseif ($_SERVER['HTTP_REFERER']){
     parse_str(parse_url($_SERVER['HTTP_REFERER'], PHP_URL_QUERY), $query);
     if ($query['value']) {
         $defaultValue = give_clean($query['value']);
     }
   }

    // we don't want to set the default value if it's the same as the field's default value
    if ($defaultValue === $field->getDefaultValue()){
      return;
    }

    $field->defaultValue($defaultValue);
});