I need a way to enforce WebService to output JSON on GET request

I wrote this:

 
Imports System.Web
Imports System.Web.Services
Imports System.Web.Script.Services
Imports System.Web.Services.Protocols

<ScriptService()> _
<WebService(Namespace:="http://tempuri.org/")> _
<WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)> _
Public Class CrossDomainFriendlyService
Inherits System.Web.Services.WebService

<ScriptMethod(ResponseFormat:=ResponseFormat.Json, UseHttpGet:=True)> _
<WebMethod()> _
Public Function HelloWorld() As String
Return "Hello World"
End Functi ...