I assume you have binded your combobox in codebehind.
Dim cmd As New SqlCommand(Query, connection)
Dim dt As New DataTable()
Dim da As New SqlDataAdapter(cmd)
da.Fill(dt)
Dim dr As DataRow = dt.NewRow()
dr("column name") = "Your default text here"
dt.Rows.InsertAt(dr, 0)
ComboBox.DataSource = dt
ComboBox.DataBind()
Regards..:laugh: