Im redesigning a webiste for an organization but in Squarespace. They need to accept membership fees on the website with the form that needs to be filled out (see the link below). I know how to create a pay button in for Paypal but don't know how to create a form for it, there is an option to chose how much you want to pay and selected amount becomes an amount to pay with Paypal. Please take a look at this link and help me if you can , how can I do something like this for Squarespace. Thank you 

https://www.obmg.org/payments/dues/