There are two ways to find your Player ID:
Option 1: Start the game. On the loading screen, look in the upper right corner for ID: ####. This is your Player ID.
Option 2: Start and enter the game. Tap on the Agent icon on the upper left to reveal the submenu. (You may need to go through the tutorial if this is your first time playing.) Then tap on the Player Profile button (the first from the left) to enter the Player Profile screen. On that screen, look for ID: ####. Again, that's your Player ID.