Show a Client Alert After the codebehind has been procesed. ASP.NET

I have this code behind in my ASP.NET 4 Web Forms app:

Protected Sub BTNSave_Click(sender As Object, e As EventArgs) Handles BTNSave.Click

   If saveOnDB() Then
      showOkAlert()
   Else
      showErrorAlert()
   End If
End Sub

I need to show a client alert when something done in the code behind has gone OK or an error has happened. Any idea?

I haved tied Response.Write("<script>alert('Successfully added');</script>") but this never works and even creates an error in the browser's console: "Uncaught Sys.WebForms.PageRequestManagerParserErrorException: Sys.WebForms.PageRequestManagerParserErrorException: The message received from the server could not be parsed."

Also I don't use OnClientClick because it triggers the javascript before the codebehind.