You can set your forum name on your profile page. Until you do this, we will use a temporary name, such as NAS12345. Your forum name should not obviously relate to your real identity, and therefore is different from the username that you created for signing in.
Make sure that you are signed in. First, go to your profile, by clicking on your "avatar" at the top right of the screen, as below, and then clicking Profile in the drop-down panel:
Then click Edit Profile, and you can type a new name into the box at the top of this screenshot:
Make any other changes that you wish, and then click Save Profile.