First of all, please make sure you already created your Steam account and have both Steam and THE GAMEs installed.
When Login to Plair Platform:
1. Click the ^ symbol next to your username on the top right corner
2. Go to [Settings] >> [Game Platform] >> [Connect to Steam]
3. A pop up will instruct you to log-in to Steam. Type your Steam username and password.
4. All set! You are ready to join tournaments!