Error trying to update panel source data

Trying to update a panel source using JavaScript on a RiSE page. Getting a 403 trying to update the data, with an error message of "Does not have required permission."

Step 1: GET /api/paneldatasource/101 - returns a 404. My assumption is that the 404 is returned because there is no data for ID 101. If I create a panel and add it to a content record and update the data, a subsequent call returns a 200.
Step 2: generate a JSON packet matching the data from an existing record (this packet works in a POST via Postman)
Step 3: POST /api/paneldatasource/ - returns a 403 with an error of "Does not have required permission."

This error doesn't occur when logged in as a full user, just a public user. My goal is to allow the user to update the data, but if I can't GET it, and I can't POST it, I have nothing to PUT - so I can't get to that step.

Thanks