If you'd like to pre-populate a Formidable form on the donation thank you receipt page this is a simple way to pass values to $_GET which it expects.

formidable-givewp-pass-values-proof-of-concept.php

<?php 

/**
 * Passing receipt values to $_GET for formidable forms.
 *
 * @see: https://formidableforms.com/knowledgebase/using-dynamic-default-values-in-fields/#kb-pass-a-value-to-a-form
 * 
 * @param $donation
 * @param $give_receipt_args
 */
function givewp_pass_args_to_get($donation, $give_receipt_args) {
	$_GET['my-param'] = isset($give_receipt_args['donation_receipt']['donation_status']['value']) ? $give_receipt_args['donation_receipt']['donation_status']['value'] : '';
}


add_action('give_payment_receipt_before_table', 'givewp_pass_args_to_get', 10, 2);