when do you want to trigger the update
you could do a dlookup, or an SQL select and assign that as part of the trigger process.
Lets assume you are looking up the area (based on telephone area code?
write a function which retrieves the area value
eg GetAreaValue(AreaCode as ????) as string
you then need to force that area code to update every time the value that contains the area changes. this will happen when you move records within a recordset, or you change the trigger value. for the former the forms "on current" event is handy, the latter the controls on chanage, on lost focus events are usefull.
if that is you problem area have a look at the help system and try to understand what events are.
If that isn't your problem then you need to consider how you define what the problem is, and how you describe that problem on this forum - by defintion we don't know your application and are not going to understand the problem in the same ligth as you see it.
Often sitting down to clearly think through the problem automatically brings a solution to mind, some of that is experience, but some is just breaking the problem into logical steps and then attempting to find a technical solution to the problem at hand.