I just signed up for a Spotify account through the main page:
https://www.spotify.com/us/signup/
I was not given the opportunity to pick a username. I was assigned a random string of charaters and numbers as my username. After speaking with Chat, they indicated that "In this case, you won't be able to choose your username."
I find it disappointing that signing up in the traditional way through the website prevents you from selecting a username, in addition to having no ability to change the username.