Similar to a form, I used an unbound text box and a button that triggers checking if the value already exists in the target table, if not adds it and requery the subform
The exclusion test is not really needed since once the item is in the exclusion table it won't be selected anymore so it does not give a chance the user to see it again but I added it for the sake of data testing.
Code:
Private Sub CmdExcludeItem_Click()
'Set variable
Dim sItem As String
Me.txtExcludeItem.SetFocus: sItem = Me.txtExcludeItem.Value
'test if value already in exclusion table
If DCount("[Item_ID]", "tbl_Requisition_Exclusions", "[Item_ID] = txtExcludeItem") > 0 Then
MsgBox "Item already excluded"
'Emtpy text box value
Me.txtExcludeItem.Value = Null
Else
DoCmd.RunSQL "INSERT INTO tbl_Requisition_Exclusions (Item_ID) VALUES ('" & txtExcludeItem & "')"
MsgBox "Item added to exclustion list"
Me.txtExcludeItem.Value = Null
'Refresh subform data
DoCmd.OpenQuery "qry_DELETE_tbl_Requisitions_Select"
DoCmd.OpenQuery "qry_AppendTo_tbl_Requisitions_Select"
Me.frm_sub_Requisition_Items_Selection.Requery
End If
End Sub