Contract testing

I have a question around testing! As of now, I'm in a unique situation where the software I am writing is going out to a third party vendor and storing some data. In this situation, writing a test to confirm that that data is posted successfully means using a production endpoint...which I doubt is a smart way to write things.

In this situation, no one seems to have a good solution for ensuring the endpoint works other then asking the third party for a dummy/test setup. I was wondering if anyone had a better idea of how to perform this sort of interaction. How do you write an efficient contract test when you don't OWN a bit of your codebase?

Please and thank ya :D