How to open a custom form in popup

I am currently working on https://jvmarketing.org . I am using the membership plugin and the theme Just Landed. The membership plugin gives a feature that if someone buys a subscription package he has to go through their subscription form. I have copied that link and pasted it into my pricing table but when the user clicks on the link from pricing table it redirects to a page where the form looks really ugly and it also says “this is an error”.

<a href=”https://jvmarketing.org/wp-admin/admin-ajax.php?
action=buynow&subscription=3″>Link for registration

The registration page

I want to make the form open in a popup just like it opens in the registration link in the pricing table

https://www.ads-software.com/plugins/membership/