Hi Preethi,
Try the below code.
Dim oComboBox As SAPbouiCOM.ComboBox = objForm.Items.Item("ComboBox Item ID").Specific
oComboBox.ValidValues.Add("", "")
oComboBox.ValidValues.Add("Y", "Yes")
oComboBox.ValidValues.Add("N", "No")
oComboBox.Select("Y", SAPbouiCOM.BoSearchKey.psk_ByValue)
Regards
Kartheek