How do I authenticate users?

Hi,
I am trying to prototype a game (my client is Unity), and I'm trying to authenticate users. The page on the subject at the main site is confusing at best, so I have no idea how to do it.

What I would kind of expected to happen was that I will be able to send the login information through the InitRequest object, however, I was unable to find any way to control it from the client and enter any information into it.

So, how do I solve this?
Thanks.