When creating a new data connection to receive data from a web
service, if that web service requires a passed parameter in order to
return a value to the form, InfoPath prompts you to specify that
parameter with the "Set Value" button.

I was hoping you would be able to specify a form field for that
parameter, similar to the way it works when defining a connection to
submit data.  However, when it prompts you for a parameter, I must
enter a literal parameter, I can't point to a form field and say "use
that value".  I have tried using both "string" and "XmlNode" types
within the web service itself.

Short version of the question: Is there any way to get InfoPath to
allow you to dynamically specify the parameter based on the form
contents by telling it to use a field's value for the parameter,
rather than simply typing it in ahead of time?