Example code for the Caldera Forms caldera_forms_submit_return_redirect filter, see: https://calderaforms.com/doc/caldera_forms_submit_return_redirect/

caldera_forms_submit_return_redirect.php

<?php
add_filter( 'caldera_forms_submit_return_redirect', 'my_redirect', 10, 4 );
function my_redirect( $url, $form, $config, $processid ){
	global $transdata;
	if ( ! empty( $transdata[ 'my_slug' ] ) && ! empty( $transdata[ 'my_slug' ][ 'url' ] ) ) {
		return $transdata[ 'my_slug' ][ 'url' ];
	}

	return $url;
}

return-url.php

<?php
	$returnurl = $referrer['scheme'] . '://' . $referrer['host'] . $referrer['path'];

	$queryvars = array(
		'cf_tp' => $processid
	);
	if(!empty($referrer['query'])){
		$queryvars = array_merge($referrer['query'], $queryvars);
	}

    	$returnurl = add_query_arg( $queryvars, $returnurl );

set-url.php

<?php
global $transdata;

$transdata[ 'my_slug' ][ 'url' ] = 'https://someapi.com?key=whatever';
return array(
	'type' => 'success'
);