Create service sales order using Manage Sales Orders with the fulfillment option set as external

Hi,

I am trying to create a sales order for a service that is coming from a third party application. I am not sure which option I have to use to have a service sales order and not a product sales order and if it's possible to select the fulfillment option as external and then put the id of the supplier.

I have noticed that FulfilmentPartyCategoryCode is there where I can set it to external, but how can I enter the supplier id?

Any help?

Thanks.