1. Find the .htaccess file for the addon domain in question. The steps for how to find it can be located here.
2. Add the following line to your .htaccess file:
<FilesMatch ".(?:html|php)$">
SetHandler application/x-lsphpXX
</FilesMatch>
For servers with LiteSpeed the code should be the following:
<FilesMatch ".(?:html|php)$">
SetHandler application/x-httpd-alt-phpXX
</FilesMatch>
where XX is the PHP version you wish to set. It can be 52, 53, 54, 55, 56, 70, 71,72, 73, 74, 80 or 81.