You just have to go to your spotify account (Thru desktop) and go to set device password. And from there you'll see a set of numbers and that is your spotify username.
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It works exactly as the dude described. In the desktop app, go to your Profile. Then...
Open anything you can paste text into and do a Paste (Command+V for max, Ctrl+V for windows). Your user name will be the number at the end of the url:
This works for me!
And what sould be my password? Thse same as the Facebook one?