Adding tasks to G Suite users using a service account

Question

I have a G Suite domain with working API access to the domain and calendars using a service account.

I'd like to be able to insert tasks in to the task lists of my users.

I'm looking at the documentation here: https://developers.google.com/tasks/v1/reference/tasklists/list

I can get "my own" tasklist like this: GET https://www.googleapis.com/tasks/v1/users/@me/lists

I'd like to get other users's list. However I get 404 error when trying this: GET https://www.googleapis.com/tasks/v1/users/foo@example.com/lists

Any suggestions on how to get a users task lists?