Refer to this link. REST Data Protocol Handler - DevTest Solutions - 10.1 - CA Technologies Documentation
The example found above refers to a GET and demonstrates how to parse parameters out of the URI. It also demonstrates how to name those parameters. In your case, the concept is the same when sending a POST operation.
If the parameters you are referring to are part of a query string, DevTest will parse those as key/value pairs and place them in the argument list using the key name identified in the query string.
If you desire to manipulate the names (key side of the key/value pair), reduce the number of elements, copy elements and values, etc., search for the Request Data Manager (RDM) Data Protocol Handler in the documentation.