Postman fails to return json from asmx service

I have WebService and I'm using Postman to test it. But it keeps returning xml. I tried using [ScriptMethod(ResponseFormat = ResponseFormat.Json,UseHttpGet =false)] in my WebMethod, also in postman the request is http://localhost:port/folder/service.asmx/HelloWorld. In the config File I Have

  <handlers>
  <add name="ScriptHandlerFactory" verb="*" path="*.asmx" type="System.Web.Script.Services.ScriptHandlerFactory,  System.Web.Extensions,  Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" resourceType="Unspecified" />
  </handlers>

It still doesn't work and Postman returns xml