The web service call asks for a method (GET / POST) and a URL that will be built by an expression. The function urlencode() may be helpful for that. Example:

"https://my.server.com/webservice/message.php?bib=" & [bib] & "&name=" & urlencode([FirstName] & " " & [LastName])

If using a POST request then you can enter the POST content, note that you do not need to escape quotation marks within the editor, it is treated as a full-text expression allowing the use of fields within the POST body. 

Additionally for web service calls the response from the call can be saved to an additional field.