When you log in to the User Interface, you use an account key like RMDEMO. But to access the API, you must do with with your account UUID.
The account UUID is needed on most API endpoints available. The first step in any API application should be to retrieve this and store this string for future use. The call to https://services.reachmail.net/administration/users/current will accomplish this. If you are using one of the ReachMail API Wrappers, the function should have a similar if not identical in name.
Full documentation here: http://services.reachmail.net/#Administration@/administration/users/current
A call to https://services.reachmail.net/administration/users/current returns the following: