You are correct, Get tasks is the method to use. You'll want to use the reference parameter to indicate the app item(s) you want to find tasks associated with. Here's an example, if you are calling the API directly:
If you use one of our client libraries you will have to use the corresponding method, but the reference syntax is the same.
Bear in mind that you must be authenticated as a user who can view these items and tasks. The Get tasks operation does not support app authentication.