How to make the Buyer Read-Only but inherit from the user in Create Order form?

Dear all,

At this moment we can select any Buyer we want in Create Order in Procurement. The requirement is to restrict this to the Buyer who is also the user who is logged in and setup as a Procurement Agent. However, it should not be possible to update this to a different buyer?

I made a Sandbox and I can't find how to accomplish this in the Create Order form. When I'm in the PO itself, I can make the LOV for Buyer as Read-Only = True, but then I'm still able to select all Buyers when creating the PO.