A combo box can never be bound to an autonumber column, i.e. its ControlSource property cannot be the name of an Autonumber. Its BoundColumn property, on the other hand, will often reference an autonumber column in the control's RowSource.
From your description it sounds like the combo box is intended to be a navigational control, with the option of going to a new empty record in the form. For this the combo box should be unbound, and code in io9ts AfterUpdate event procedure should move the form to an empty new record if the appropriate option is selected in the control, or go to an existing record otherwise. You'll find an example in FindRecord.zip in my public databases folder at:
https://onedrive.live.com/?cid=44CC60D7FEA42912&id=44CC60D7FEA42912!169
In this little demo file an unbound combo box in a contacts form has the following as its RowSource property:
SELECT ContactID, FirstName & " " & LastName,
1 As SortColumn, LastName, FirstName
FROM Contacts
UNION
SELECT 0, "<New Contact>", 0,"",""
FROM Contacts
ORDER BY SortColumn, LastName, FirstName;
The UNION operation adds the <New Contact> option to the list, in addition to the list of contact names. The code for the control's AfterUpdate event procedure is:
Private Sub cboGotoContact_AfterUpdate()
Const MESSAGETEXT = "No matching record"
Dim ctrl As Control
Set ctrl = Me.ActiveControl
If Not IsNull(ctrl) Then
If ctrl = 0 Then
' go to new record and move focus to FirstName control
DoCmd.GoToRecord acForm, Me.Name, acNewRec
Me.FirstName.SetFocus
Else
With Me.RecordsetClone
.FindFirst "ContactID = " & ctrl
If Not .NoMatch Then
' go to record by synchronizing bookmarks
Me.Bookmark = .Bookmark
Else
MsgBox MESSAGETEXT, vbInformation, "Warning"
End If
End With
End If
End If
End Sub