How to I add an MDM for use in business manager without a customer number?

My organization has had a large number of iPads for quite some time - all of which were managed using a physical connection and Apple Configurator. We have decided to use an MDM for various reasons.

I was trying to load each device into Apple Business Manager to then assign them to our MDM of choice. Unfortunately it doesn't seem as though I can add the MDM server in ABM (because the box for "Add new MDM server" is greyed out) without a customer number or reseller ID entered. The site prompts me for either of those pieces. We simply don't have those numbers as we got the devices two or three years ago from an arms-length parent organization who manages purchasing and is not technically a reseller nor were we provided an invoice. It is akin to them being donated devices.

How can I go about adding the MDM?

Thanks