Force redirect not logged in user to (wp-login.php or wp-admin) for specific page

I am currently using a WordPress plugin (Pagerestrict plugin which references URLS based on Page ID/Slugs)to restrict certain pages to logged in users only. This plugin works fine with standard wordpress URL structure but the issue I have is some of my pages which are directly linked to some plugin functionality have http://mywebsite.com/wp-content/plugins/purchase.php instead of a standard http://mywebsite.com/wp-content/plugins/purchase/ wordpress url structure. So what i tried in cases like this is to manually add the functionality below which didn’t work

AJAX form post returns 0

I am working on a simple plugin that shows an HTML form where the user can input a keyword which is used to query an external API. I currently have the form and the related jQuery set up, but when passed to the plugin’s PHP file, I get a response of simply 0. At this point I’m only just trying to get any response but 0 (see the echo "Whyyyy" part). I’ve been scouring the internet and feel like I have tried everything that’s been suggested, leading me to think I’m making a mistake somewhere.