What code is needed to create an email and access code form linked to another site?

I am building a website for my company through Squarespace. We are a photography company who uses Photo Reflect. As of now, are customers are going to our Photo Reflect site to log in, access their photos, and purchase additional products. I am seeing if there is a way or code needed to have customers log in with their email and access code through Squarespace, and when they hit submit, they will be re-directed and already logged in to their Photo Reflect account. Is this possible?