Partner Portal - Device Deployment API - Purchase Order ID

Question

I am testing out uploading devices into customer tenants using the the UI in the Partner Portal, the PartnerCenter PowerShell Module and the API. I can see that there's an option to add a BatchID which turns into the Group Tag when a device is loaded in the customer's Autopilot portal, however I can't see how I can specify the PurchaseOrderID for a device. I contacted support and they didn't seem to know and suggested it could be an OEM only option, but without access to the OEM Direct API or docs, I can't confirm that. I know this is possible since I have devices that are being loaded into other tenants that come in with PurchaseOrderID filled in, but no Group Tags. Can anyone point me in the right direction?