Rewriting subfolders to specific parent folder in Wordpress

I have created one page with name /parent/ and assigned new template template-parent.php that has some 3rd party software showing via JavaScript. I need to rewrite all subfolders to this parent so when someone tries to access

domain.com/parent/something

the url will remain the same, but he will be pointed to domain.com/parent/ so that loaded scripts could process the request.

The code should be something like this

RewriteRule ^(parent)/(.+)$ /$1/ [QSA,L]

but I am not sure where to put it, should I use .htaccess or put it in functions.php If in htaccess, then will it be before, after or in the middle of wordpress code.