Simple custom redirect hook for WordPress with domain whitelisting. Simply download the file

custom-redirect-trigger.php

<?php
/**
 * Plugin Name: Custom Redirect Trigger
 *
 * This plugin allows all redirects to be processed. It can be dangerous if used incorrectly. This includes
 * not adding a whitelist domain redirect. Having this wide open will all hackish redirects.
 */

class Custom_Redirect_Rewrites {
  
  /*
   * Add domains without http or https in the array below. only use the host. no exstensions or prefixes.
   */
	public $whitelisted_redirects = array(
		'allowed-domain.com',
    'allowed-domain2.com'
	);

	function create_rewrite_rules( $rules ) {
		global $wp_rewrite;
		$newRule  = array( 'crt/(.+)' => 'index.php?crt=' . $wp_rewrite->preg_index( 1 ) );
		$newRules = $newRule + $rules;

		return $newRules;
	}

	function add_query_vars( $qvars ) {
		$qvars[] = 'crt';

		return $qvars;
	}

	function flush_rewrite_rules() {
		global $wp_rewrite;
		$wp_rewrite->flush_rules();
	}

	function template_redirect_intercept() {
		global $wp_query;
		if ( $wp_query->get( 'crt' ) && $wp_query->get( 'crt' ) == 'redirect' ) {

			// Check the provided redirect against the whitelisted redirect
			$redirect_uri = ! empty( $_GET['redirect_crt_uri'] ) ? $_GET['redirect_crt_uri'] : false;

			if ( ! $redirect_uri ) {
				exit( 'No redirect provided' );
			}

			$redirect_host = parse_url( $redirect_uri, PHP_URL_HOST );
			if ( ! in_array( $redirect_host, $this->whitelisted_redirects ) ) {
				exit( 'Unauthorized Redirect URL' );
			}

			wp_redirect( $redirect_uri );
			exit;
		}
	}
}

$WPOSSO_Rewrites = new Custom_Redirect_Rewrites();
add_filter( 'rewrite_rules_array', array( $WPOSSO_Rewrites, 'create_rewrite_rules' ) );
add_filter( 'query_vars', array( $WPOSSO_Rewrites, 'add_query_vars' ) );
add_filter( 'wp_loaded', array( $WPOSSO_Rewrites, 'flush_rewrite_rules' ) );
add_action( 'template_redirect', array( $WPOSSO_Rewrites, 'template_redirect_intercept' ) );