The title says it all.
Hey, Did you find out ? Where to find it?
maybe this helps http://www.virtualonlinecycling.com/p/zwiftid.html
Thanks a lot.
On Android you can do it like this:
Go to my.zwift.com in e.g. Chrome, pick any of your activities, and long press on the download link. The user ID is part of the URL to the .fit file, right after where it says ‘prod/’.
If you cannot see the full URL in the popup menu (it may be cut off depending on the screen resolution of your device) then simply choose ‘Copy link address’ and paste the link address into the address bar of the browser. Then you can see the full URL and locate your user ID in it.
Here are described other different ways to find your ID without having to share your username and password with anyone: