hook_wp.php

<?php
add_action( 'wp', 'wphc_redirect_url' );

function wphc_redirect_url() {
  
	if ( ! is_singular() ) {
		return;
	}
  
	$redirect = get_post_meta( get_the_ID(), 'redirect_url', true );
	if ( ! empty( $redirect ) ) {
		wp_safe_redirect( $redirect, '301' );
		exit();
	}

}