How to Retrieve the Message ID in a BPM

I have read several post on retrieving the message ID in a BPM and I'm still not clear on the required steps. From what I've read it appears that within the process the message ID is not available to the transformation step.

However, in one thread mentioned that "the target message of the sender would have to be modified to store the message. Then within the Map assign this field using the system constant at runtime."

I'm really not clear on the steps that I should be take to retrieve the message ID. In my BPM the first step is the Recieve step where we recieve the data into a message interface and there is no mapping required. Afterwards you are within the BPM which, from my reading, means that the message ID is no longer availabe. We then have a transformation step and then a Send step to send the data out and once again mapping is not required.

My question is based on the above process how would I retrieve the message Id?