Your Support ID is the same as the Invitation Code you use to add friends.
You can find your Support ID in the game settings by pressing the gear button in the top right-hand corner of the screen.
If for some reason you cannot get into the game's settings, you can see your Support ID:
- in the top left corner of the loading screen (Android, Amazon, and Windows).
- in your device's settings: Settings - Seekers Notes - Support ID (iOS).