Display MessageBox From ServerSide

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    Dim message As String = "Hello! Mudassar."

    Dim sb As New System.Text.StringBuilder()

    sb.Append("<script type = 'text/javascript'>")

    sb.Append("window.onload=function(){")

    sb.Append("alert('")

    sb.Append(message)

    sb.Append("')};")

    sb.Append("</script>")

    ClientScript.RegisterClientScriptBlock(Me.GetType(), "alert", sb.ToString())

End Sub