The hubspot_owner_id property is the default property that manages contact ownership. This property takes an owner's ID, which you'll need to use the Owners API to get:
Great! What was the solution? I’m hitting this problem now.
I’m creating a new contact in a custom application and at the same time creating it in HubSpot… works great! Except that I can’t set the owner in HubSpot. So the custom app has an owner, but the HubSpot account does not have an owner.