Setting Seller in SalesOrder

Hi,

I must set the seller during creating a new SalesOrder in my programm (Field Seller see screenshot). I have found in ReposirtoryView in SalesOrder object an assosiation to Seller, but it is empty and I'm no be able to set the reference to Party here (Seller is read only). I suppose I have to add a Party with role "Seller" to node "Party" but I'dont know how I can do it.

Have anybody an Idea?

Best regards,

Boguslaw