How do i add a Facebook like pop up box to squarespace homepage?

If you are referring to the actual web builder Squarespace, then you might need to contact them if they have a widget or plugin for a Facebook like function.

If you are referring to how to add a Facebook like option page in your JotForm form, then that should be possible by adding this widget in your form: https://widgets.jotform.com/widget/facebook_like_box. To add a widget to your form, you can follow this guide: https://www.jotform.com/help/252-How-to-Add-a-Widget-to-your-Form 

If you are embedding the form to your Squarespace website, you can also follow this guide: https://www.jotform.com/help/37-Adding-Form-to-Squarespace 

I hope that helps.