How do I replace System.Web.UI.Page RegisterStartupScript?

I've got old code that I am working on, and I'd like to replace some depreciated methods like this one:

    Public Shared Sub cmnCloseForm(ByRef pgTemp As System.Web.UI.Page, Optional ByVal sMsg$ = "", Optional ByVal sExtraJava$ = "")

        ' Variables
        Dim sJava$

        ' New Variable
        Dim sBld As StringBuilder = New StringBuilder()

        ' Create Java String
        sJava = IIf(sMsg <> "", "alert('" + sMsg + "'); ", "") + _
                IIf(sExtraJava <> "", sExtraJava, "") + _
                "self.close();"

        ' Append Strings
        sBld.Append("<script language='javascript'>")
        sBld.Append(sJava)
        sBld.Append("</script>")

        ' Register String
        pgTemp.RegisterStartupScript("closing", sBld.ToString)


    End Sub

That creates a dialog box on the web page that looks like this:

I tried replacing it with ScriptManager.RegisterStartupScript:

    Public Shared Sub cmnCloseForm(ByRef pgTemp As System.Web.UI.Page, Optional ByVal sMsg$ = "", Optional ByVal sExtraJava$ = "")

        ' Variables
        Dim sJava$

        ' New Variable
        Dim sBld As StringBuilder = New StringBuilder()

        ' Create Java String
        sJava = IIf(sMsg <> "", "alert('" + sMsg + "'); ", "") + _
                IIf(sExtraJava <> "", sExtraJava, "") + _
                "self.close();"

        ' Append Strings
        sBld.Append("<script language='javascript'>")
        sBld.Append(sJava)
        sBld.Append("</script>")

        ' Register String
        Dim page As Page = CType(HttpContext.Current.CurrentHandler, Page)
        ScriptManager.RegisterStartupScript(page, page.GetType(), "closing", sBld.ToString(), True)


    End Sub

The new code above does this (notice the text added to the dialog box):

How do I correctly replace this depreciated code?

I simplified the code and used ClientScriptManager.RegisterStartupScript, which seems to be made to replace the deprecated method.

Here is my resulting code:

Public Shared Sub cmnCloseForm(ByRef pgTemp As System.Web.UI.Page, Optional ByVal sMsg$ = "", Optional ByVal sExtraJava$ = "")

    ' Variables
    Dim sJava As String
    If String.IsNullOrEmpty(sMsg) Then
        sJava = String.Format("{0} self.close();", sExtraJava)
    Else
        sJava = String.Format("alert('{0}'); {1} self.close();", sMsg, sExtraJava)
    End If

    ' Register String
    pgTemp.ClientScript.RegisterStartupScript(pgTemp.GetType(), "closing", String.Format("<script language='javascript'>{0}</script>", sJava))

End Sub