Passing API Key to front end

So I have a file on the server that stores a API key. So when a user logins in if a condition is met I read the key and pass it forward with the login information to be stored on in a js var. Is this a good way to do pass the key so only a login user with permissions can use that AP?. I'm using C# with .net back end and ajax.

Is There a best practice or stranded way of doing this?