If your RowSource is a Value List, the following code demonstrates how to add
a value thru VBA, where the control TextBoxValueToAdd holds the new item to
be added:
Private Sub AddValueToComboboxButton_Click()
If Not IsNull(TextBoxValueToAdd) Then
YourComboBox.RowSource = YourComboBox.RowSource & ";" & Me.
TextBoxValueToAdd
End If
End Sub