I have a Look up table for City's. On my form I attached this Event to Not in List. City is a combo box.
This event will tell user the city tey typed in is not in the list and give them the chance to add it using another entry form created. You can omit that part.
Private Sub City_NotInList(NewData As String, Response As Integer)
If MsgBox("Do you want to add '" _
& NewData & "' to the items in this control?", _
vbOKCancel, "Add New Item?"

= vbOK Then
' Remove new data from combo box so control can be requeried
' after the AddCity form is closed.
DoCmd.RunCommand acCmdUndo
' Display form to collect data needed for the new record.
DoCmd.OpenForm "frm_AddCity", acNormal, , , acAdd, acDialog, NewData
' Continue without displaying default error message.
Response = acDataErrAdded
Else
Response = acDataErrContinue
End If
End Sub