I've done a bit of work similar to what you are looking for.
I create a form that is not bound to anything (frm1), then create a text box on that form (txtBox1) that I use to input the number to be looked up. You can set this to be referencing a drop down list and define it, so the autonumbers can be typed in or selected by the drop down.
Then I add a command button to the form (cmdOpen) that opens the new form (frm2) with items that I want to view related to that number (autoNum). I place this code on that command button:
Private Sub cmdOpen_Click()
On Error GoTo Err_cmdOpen_Click
Dim stDocName As String
Dim stLinkCriteria As String
stDocName = "frm2"
'Closes the form frm1 if there is no input,
'else it opens the form frm2.
If IsNull([txtBox1]) Then
DoCmd.Close acForm, "frm1", acSaveNo
Else
stLinkCriteria = "[autoNum]=" & Forms![frm1]![txtBox1]
DoCmd.OpenForm stDocName, , , stLinkCriteria
End If
Exit_cmdOpen_Click:
Exit Sub
Err__cmdOpenClick:
MsgBox Err.Number & " : " & Err.Description
Resume Exit_cmdOpen_Click
End Sub
Tony L
ABQ, NM