We could use the below htaccess code to redirect particular URL from HTTP to HTTPS.
I the below code, we have added redirection for “donate_wwp.php” payment page from HTTP to HTTPS.
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteCond %{REQUEST_URI} donate_wwp.php
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
NOTE: I have added it top of the htaccess file.