Fetch API problem

JavaScript
1

Hi:
Because my API server can not input and output startard JSON format, (use javascript object)
Can some one tell me, way can’t get value ?

      function login(){
        var obj3={wa99:"{\"class_name\":\"member\",\"function_name\":\"siteStatus\"}"};
        // fetch API 
        fetch('https://tstgen.tg7777.net/iapi/post.php',{
            headers:{
                'Content-Type':'application/x-www-form-urlencoded'
            },
            method:'post',
            body: {wa99:"{\"class_name:\"member\",\"function_name\":\"siteStatus\"}"}
        }).then(function(response){
            return response;
        }).then(function(value){
          console.log(value);
        });
      }

if I use jquery like the

        var obj3={wa99:"{\"class_name\":\"member\",\"function_name\":\"siteStatus\"}"};
        var settings = {
          "async": true,
          "crossDomain": true,
          "url": "https://tstgen.tg7777.net/iapi/post.php",
          "method": "POST",
          "headers": {
            "Content-Type": "application/x-www-form-urlencoded"
          },
        "data": obj3
      }
      $.ajax(settings).done(function (response) {
          if (response.errorCode=='000'){
             console.log('OK');
          }
          else {
           
          }
      })

I will get value very easy, How can I do , if I want use fetch() api ?