Sorry to hear you’re having trouble here. A users My Tasks list is essentially like any other project in Asana’s data model, so you can absolutely create a webhook on it.
For your resource ID, you’ll want to use the My Tasks list ID. You can find that by visiting a user’s My Tasks list in Asana and checking the address bar:
https://app.asana.com/0/<my-tasks-id>/list
Let me know if that helps or if you’re receiving any errors when you try to create this webhook.