Thank you page after donation or paid subscription

Site URL: https://www.searchangels.org/contribute

In almost every other form platform you can send/redirect people to a "thank you" web page after the sale is confirmed. I already send them an email, which is already built into Squarespace. I am unsure if this requires a special CSS hack or a Code Injection. I have developer mode enabled, but the original template was "Bedford". The account level if advanced commerce; which enabled paid subscriptions. We only use purchases through Squarespace for donations or paid subscriptions. Any help is welcome.