Your Bubble App ID is the unique name you have given to your app when you created it. You can find it a few places, including:
In the URL of your app editor, just after
id=For example: bubble.io/page?name=index&id=yourappid&tab=tabs-1)
If you don't have a custom domain setup on your app, you can also find it in your preview URL before the
.bubbleapps.iodomain.For example: yourappid.bubbleapps.io/version-test?debug_mode=true