Help with htaccess redirect

I have WordPress installed to a sub directory /public_html/prelaunch/ and that sub directory defined as a sub domain, so user entering [login to view URL] will have access to the WordPress site. It works fine. However, when I try to do a htaccess redirect from [login to view URL] to a PHP file under public_html/prelaunch/custom/[login to view URL], it does not work. Instead, it gets picked up by WordPress and forwards user to WordPress 404 error page. The htaccess file is attached. The referenced PHP file exists in the server in the mentioned directory. Can you fix the problem? You will not be given server access.