AJAX action through direct link

Question

Use WordPress’ generic POST/GET handler wp-admin/admin-post.php

Don’t let the name confuse you – it accepts both GET/POST, and just like admin-ajax.php it accepts both authorised (admin) and non-authorised (public) requests.

function wpse_406199_custom_action() {
    // Do something!

    // Maybe redirect back to where they came from?
    wp_redirect( wp_get_referer() );
    exit;
}

// Non-logged-in requests
add_action( 'admin_post_nopriv_wpse_406199_custom_action', 'wpse_406199_custom_action' );

// Logged-in requests
add_action( 'admin_post_wpse_406199_custom_action', 'wpse_406199_custom_action' );

And to get the URL:

$action_url = admin_url( 'admin-post.php?action=wpse_406199_custom_action' );