Can I embed a form into the Checkout page on squarespace?

If squarespace allows the embed code in their checkout page, then you can use the embed codes of Jotform. Please try the following embed codes and see if it works on the checkout page.

https://www.jotform.com/help/34-embedding-a-form-to-a-web-page

https://www.jotform.com/help/148-Getting-the-Form-iFrame-Code

This guide might also help in adding a form into your Squarespace site https://www.jotform.com/help/37-adding-a-form-to-squarespace.