Sub CustomersGridView_RowDataBound(ByVal sender As Object, ByVal e As GridViewRowEventArgs)

  If e.Row.RowType = DataControlRowType.DataRow Then

    Dim DropDownList3 As DropDownList = DirectCast(e.Row.FindControl("DropDownList3"), DropDownList)
    Dim btnTest As Button = DirectCast(e.Row.FindControl("btnTest"), Button)
    
    btnTest.OnClientClick = String.Format("alert(document.getElementById('{0}'));", DropDownList3.ClientID)

  End If

End Sub