Login Portal from Another Site

Hi! 

I'm trying to add a login to my client's Squarespace account. They want their clients to be able to enter their login credentials (Username and PW) on their SQSP website, but then when they hit Submit, they would be taken to their current members site that's hosted on another platform. Is this possible? I'm thinking I'd have to embed the Member site's login form into a Squarespace page but I'm unsure as to how I would do that. ...