if you ask me for my favorite tf2 class your blocked instantly
It's a fake competitive league that they ask you to register in with your steam account.

After that, the window where you enter your steam guard code hijacks your profile's information, along with the code and within seconds, you get notifications on your phone that the email, password and steam mobile authenticator device have been changed. With that, they hijack your account and lock you out and then either trade the items to their own account or sell the account as a whole (2nd one is more likely).