Using Microsoft Dynamics GP Web Services, how do I add a salesperson to a sales invoice?

Question

I need to add a salesperson to a GP sales invoice. I'm using the web services command CreateSalesInvoice. The code to create the invoice has been working for years, but I can't figure out a way to add a salesperson. The only salesperson field I could find is under commissions, so I focused on adding a record there without success. I also tried filling in a user defined field, but that also didn't work.

1. Adding a salesperson worked (the invoice was created; I created a territory and a salesperson in GP first), but the salesperson does not appear on the commissions area of the invoice in GP.

       SalespersonKey spk = new SalespersonKey
       {
           Id = "BobSmith"
       };
       salesInvoice.SalespersonKey = spk;
   

2. Tried to add a commission, but it seems to be read only.

   salesInvoice.Commissions = 
   

3. Adding a user defined field throws an error saying UserDefined is null

   salesInvoice.UserDefined.Text01 = "Bob Smith";