¿How can I deliver a delivery order that is ready to deliver using the XML-RPC API?

This is what I have tried so far:

When you activate developer mode you can see that the Deliver button in Delivery Order forms (when the delivery order is ready to deliver) refers to object "stock.picking.out" and method "action_process".

But if I call exec_workflow method of XML-RPC API with parameters "stock.picking.out" and "action_process", it always returns False and the delivery order does not change. This is the call I make from VB.Net:

res = proxyObject.exec_workflow(databaseName, userId, password, "stock.picking.out", "action_process", deliveryOrderId).

¿Should I use another parameter instead of "action_process"?

I have also tried with the execute method of XML-RPC API with the parameters "stock.picking.out", "action_process" and this list of arguments:

        Dim argId(2) As String
        argId(0) = "picking_id"
        argId(1) = "="
        argId(2) = deliveryOrderId

        Dim list As New List(Of Array)
        list.Add(argId)

        res = proxyObject.execute(databaseName, userId, password, "stock.picking.out", "action_process", list.ToArray())

But it returns a dictionary with "res_model" = "stock.partial.picking" and the delivery order does not change either.

Thanks.