I connected YouTube in Streamlabs. How will Playbox show up on my stream?

It won’t: if YouTube is linked inside Streamlabs (or you use YouTube’s Go Live), Playbox doesn’t know about that broadcast, so the game has no stream in the lobby and the video has no scoreboard. Connect the stream through Playbox instead:

1. In the game tap “Add stream” (lobby, “Media”, “Video”, or “Manage”, “Stream settings”). Choose your channel under “Your channels” (connect it first in Settings, “Connected Apps”, YouTube) or “Playbox channel”, tap “Next”, pick the quality and tap “Create”.

2. Tap the stream row, then “Copy stream key” and “Copy RTMP server”.

3. In Streamlabs choose Custom RTMP instead of YouTube and paste both.

4. Add the overlay link from Stream settings as a layer: Scenes, Layers, Add, Add Custom Item, Add URL.

Then the stream appears in the game lobby, and the video shows the live score recorded on the phone that runs the match.