In the Allstar app, go to App Settings > Games > Set Hotkey. From there, click into the hotkey box, select the key or key combo you would like to use, and hit Set!

All Chat:
!allstar will appear in All Chat for everyone to see.

Team Chat:
!allstar will only appear in the Team Chat for your teammates.

None (Doesn’t say !allstar):
!allstar will not appear in any chat, but you will still hear the Allstar cheer.