Use the in-game reporting tool.
To use the in-game reporting tool, open the scoreboard, tap on the player's name, and select 'Report Player.'

If you want to report a hacker manually, we need proper video footage to prove the user's offense.
Upload your video as hidden to YouTube or Google Drive and post the link to the video for us.

You can either send the link to us through the in-app help tool:
Open the game → Settings → Help → Choose any topic → "Not helpful" → Contact us button.

Or through the web chat widget, which you can see bottom right of the screen on this FAQ page.