pre-fill-with-url-parameters.php

<?php

function my_give_tribute_by_default() { ?>
	<script>
		let searchParams = new URLSearchParams(window.location.search);
		// Change the parameter name here
		let referrer = searchParams.has('referrer') ? searchParams.get('referrer') : '';
		// Change the target variable here
		jQuery("input#ffm-referrer").prop("value", referrer);
	</script>

<?php }

add_action( 'give_donation_form_after_submit', 'my_give_tribute_by_default' );