When you are registering in the Mage2.PRO repository you set your password and use your email (specified in the registration email) as your login:
But you need to know your Mage2.PRO username to setup the Composer authentication in the Mage2.PRO repository:
From 2016-09-02 the recommended way is to use an SSH key authentication instead of the password authentication
Step 1
Go to the Profile settings of your Mage2.PRO repository account:
Step 2
Select the Account menu item in the left sidebar and then scroll the page down to the Change Username section.
You will see your username. Use it for the Composer authentication.
Alternatively, you can setup a SSH key for the Composer authentication.
