You will need to use your Steam account to register/login into our forums. This is what you should see:
Click on the Steam button. A popup will display. Enter your Steam account informations and you should be registered. Note that everything is managed by Steam, we only have access to player name and profile picture.
If the Steam button isn't displayed, remove your ad blocker if you have one. Alternatively, use another browser such as Edge, Chrome or Firefox.
A firewall could also block the popup.
If nothing fixed the login problem on your computer, try with your phone!