how to add additional session variable with Master details set

after log in i am using MM_Username to pass the username to all following pages successfully via a

1/.log in page

2/. 3/.Master page & details page  (All data in one table 'members')

On the details page i have a link to a 'message' table

I can pass the MM_username no problems into the message table and the message

I want to also insert the username (or id) for the record that the details page is displaying.

So when i submit i will have

The Username of the person viewing the details (From MM_Username)

The username of the record that is being viewed

and the Message.

So how do i add a new session variable for the record being viewed and insert it into the message table ?