I have a function as follows:
https://developers.arcgis.com/rest/users-groups-and-items/add-item/#description
This operation is available only to the specified user.
I can't see anything to support that you can call this for other user accounts?
Going to the ArcGIS Online > Organization > Members > Manager Member's items, I don' that the UI to add items on behalf of that user. The response you see also indicates it's not possible.
Can you change the workflow to add the item to your user account and then change ownership?