Is there a way to create a popup form on a squarespace website?

I would like to create an email subscription form for a page on my website that pops up when someone lands on the page and they could manually click off. 

It is possible to create a lightbox pop-up form using JotForm. Please refer to the gudie below that can help you with this:

https://www.jotform.com/help/220-How-to-Create-a-Lightbox-Form

Generally, we recommend using iframe embed method to the forms on a SquareSpace website to avoid conflicts with the scripts.

Please get back to us if you need any further assistance. We will be happy to help.