Test combobox entry to populate next textbox in same form

I have a form using a combobox and several text boxes. I initialize these boxes to populate the combobox. Is it possible to test the users selection from the combobox to populate the next entry textbox? The reason is to test the combobox users selection and place a few characters into the next textbox, which in this case is the start of the serial number. For instance, if the user selects "Directional" from the combobox, I want to place "DIR" into the following text box to encourage the user to enter the serial number in the proper format DIR123
I think I know how to get the characters into the text box and then set it as the focus, but not sure where to do the test on the combobox entry...