Say your Dropdown is bookmarked "Name" and yoru text field for the Unit Identification Code is bookmarked "UIC." You need to set a macro to run on exit from the DD.

Sub NameDDOnExit()


Dim oFF As FormFields
Set oFF = ActiveDocument.FormFields
Select Case oFF("Name").Result
Case "TOPEKA"
oFF("UIC").Result = "12345"
Case "PASADENA"
oFF("UIC").Result = "54321"
Case "HELENA"
oFF("UIC").Result = "99999"
End Select
End Sub