Whenever you look up BusTracker results, instructions on what to text appear along with those results (with the stop ID shown in context).
Go to the BusTracker and get estimated arrival times route/bus stop and you’ll see instructions for what to text. Like this (click image for larger view):
Note: If you have a mobile device with Web access and want to look up a stop ID, this works in the mobile version of BusTracker, too. Simply look up a result, and instructions (including the specific stop ID) will appear with the estimated arrival times: