Authenticating SquareSpace Request via Hash or really any mechanism

So I developed a solution that is hosted on Azure. This service dynamically creates images based on user input. I want to make sure that others don't embed my link into their page. Basically I need a mechanism to authenticate HTTP requests on my server to ensure they came from my SquareSpace website. My initial thought was that I could use a shared secret, like it used for the API integrations but I don't know how to get SquareSpace to hash something and stick it in the front-end for me.

How can I get something hashed or is there a hash value that is already included in some variable some place that I can send along and verify?