Which button-enabled popup forms do you use?

I'm using the cool Agency site by Mike Oliver, on a pre-owned medical equipment website I'm building.
Each device for sale will have its own post. At the top, I'd like to insert (inside the content) a button which pops up a purchase inquiry form.

My question: Which popup solution would you use in conjunction with the site's given wpforms plugin? My requirements are:
1. minimal impact to site speed by loading too many js.
2. ability to insert event js codes when clicking on the button, and when submitting the form.

I don't mind ditching wpforms if there's a single solution that takes care of both.

I'd rather avoid mailchimp.

Anyone? :)
Thanks.