Determine if a user exists through the API?

I know I can use that to get the information of a user that’s already logged in, but I need to determine if the name they enter into a different field is an actual user. I won’t have access to the other user that they enter because they aren’t the one who logs in. Trying to make that request will just get me an Unauthorization error.

I guess for a better understanding you can try it out here.

Go to Add Stream at the top right and enter a user that doesn’t exist. It will still add that but nothing will actually work since that broadcaster never actually existed. I’m trying to catch this error beforehand rather than making the user have to go and remove the stream manually.