To retrieve, with admin access using only the task ID (which I obtain from a webhook for completed tasks), the details of a task belonging to any user. However, the API call using Service Account Keys doesn’t seem to yield any results. On the other hand, using personal authorization doesn’t allow me to access all users’ tasks.
Additionally, it doesn’t seem possible to fetch custom fields, which is crucial for me! I’d rather not waste time creating systems with XML, which are being phased out.
The XML-RPC API has the flexibility to do what you want, but we do not know when they plan to sunset it. Keap need to be do a lot more work with the REST API, including being able to query Custom Fields. Otherwise there is going to be gaps with the REST API that will make it difficult to transition across in the future.