I am creating a BPL using the Management Portal instead of creating a custom class. 

As per the documentations 'request object' contains properties those were in the original request message object or primary request and can be reference using the dot syntax. 

So if the primary request was an HL7 message, i should be able to pull the value MSH:9.2 as  follow 

request.{MSH:9.2} and assign it to a context property

attached image is a screen shot of my configuration, set  context.X  to request.{MSH.MessageType.