I would like to call a web service using a soap request, but in this case I want to use the UserTask "Call Web Service" for that. Anyone have some sample of how to build it?
You can find detailed instructions on how to call any web services from a business process in the Calling web services from business processes Academy article. Also, you can take a look at the Studying Web service documentation article that describes how to call the web service and what kind of response it will return.
Please, let us know in case any further information is required.