How to get my session api_key first before unit testing REST endpoints?

How can have my project get the session key and store it before calling the other endpoints?

Info about project: I have a list of endpoints that I would like to unit test but they require a session key as a header input.

I also have an endpoint that will provide that session key with the correct credentials.

Extra information: Project built from http://ift.tt/1NZdROV with express