After update of a field to change the value in another field

I would like to write code in an option group for an after update. When the option is updated... i would like it to put a value in another field labeled txtCallName.

The value comes from a query called qryWhosComputer. That query is a 1 row with 3 columns the third column is titled "Last". I would like that "Last" value to be entered in the txtCallName. I am guessing the code would include a DLookup of qryWhosComputer for "Last". Not sure exactly how to write it.

Thanks!