After you have integrated the iOS SDK into your game (follow the appropriate guide to do that), you can try out whether it works yourself before submitting it to TestFlight.
Launch the game build with the SDK integrated on a real iOS device or with the simulator. A locks screen should show up that prevents you from accessing the game:
To get past the lock screen, retrieve the Apple reviewer credentials for your SDK key from PlaytestCloud:
Click on Settings in the menu on app.playtestcloud.com
Click on Manage iOS SDK credentials
Look up the username & password tied to your SDK Key
Enter the username and password
The game should launch normally now, and you should be able to play it as usual.
During a playtest, players will use the PlaytestCloud App to record their gameplay. The PlaytestCloud App will continuously communicate with the SDK built into your game. It will only permit players to proceed past the lock screen when they are recording and the playtest is running, otherwise, it will prevent them from launching the game.