Hey @Daniel_Lusk,
Thanks for reaching out. I’m sorry to report that you’ve uncovered a shortcoming in our API. We hope to add @mentions and MyTasks to the API in the nearish future, though the work hasn’t been scheduled yet.
MyTasks are essentially a project, so the URL for your MyTasks is https://app.asana.com/0/<project-id>
The only somewhat practical workaround I can think of would be to create a project in Asana that maps user info (such as name, id or email) to their MyTasks project id. You could then reference this project via the API when you need to @mention a user.
There’s another possible workaround, however, it’s exceedingly gnarly. You could make GET requests for projects/<user-id +1> and then continue to increment the user-id by one. One of your first successful requests is likely to be the user’s MyTasks as this project is created soon after the user id is created (though you will likely hit their Personal Projects MyTasks first). This method won’t work if a user is added to a workspace after they were already an Asana user.