Redirect a file that is not in the hierarchy of wordpress

Good morning, I created a personal file, which is not in the file hierarchy of WordPress, this file is called reserva.php. In this file, do an include wp-load.php functions to use WordPress. The problem is that access to that file is by https://www.miweb.com/wp-content/themes/mitheme/cart/reserva.php and I would like to go through https://www.miweb.com/cart/reserva. I tried modifying the htaccess and adding the following rule:

Redirect 301 https://www.miweb.com/wp-content/themes/mitheme/cart/reserva.php https://ww.miweb.com/cart/reserva

but this rule does not work. Could someone tell me how I can solve this problem?

Thank you!