Your Zwift ID is a unique number assigned your Zwift account. It is used by ZwiftPower and other third-party apps who connect to your Zwift data. Here are two ways to find your ID…
Your Zwift ID can be found in the desktop version of Zwift by going to Documents->Zwift->CP. The folder names here (eg, “user123456” use the ID’s of every Zwifter who has ever used your computer.
If you see multiple folders and are unsure which one belongs to your account, look at the dates of the files inside the folder, which will correspond to dates you have used Zwift on this computer.