tName would contain the name of the person that handled that transaction so just putting [tName] would trigger a record for that name.
Unfortunately we can't create events out of the air. There is an event called GUILDBANK_UPDATE_WITHDRAWMONEY but not one called GUILDBANK_UPDATE_WITHDRAWITEM.
If you want to read up on what is currently available - they usually keep it well updated - visit
http://www.wowwiki.com/Portal:Interface_customization. It is what I use most of the time and definitely in the early stages of addon development. Another good one I use alot is
http://wowprogramming.com/. Both sites together will demonstrate the possibilities.
Hopefully at a later date Blizzard will work on these events so that they actually hold information for you to work with instead of having to go around the houses.
And, I hope this addon code will start you off to investigate things further and get a workable addon for what you want until they do fix things there.