The help guide or some of the sample on my site would help. Probably the best is [url]http://www.biztalkgurus.com/Samples/Untyped-Documents-Promoted-Orchestration.html[/url]. It will show you how to set message context properties inside an Orchestration.
Basically, inside a message construct shape you would have something like this:
Out = In;
Out(SOAP.Username) = “something”;
SOAP.Username can be any property you want to set – that isn’t read only.[/url]