Http Post Request Data not Correct

i use http post method for auth but its not work, in get method every things work good but in Post not
this is my code :

         this.body = {
            username: username,
            password: password
         let headers = new Headers;
           headers.set('Content-Type', 'application/x-www-form-urlencoded');
        return,JSON.stringify(this.body),{headers: {'Content-Type': 'application/x-www-form-urlencoded'}});

and this screen shot show that data not correct format

why data format not correct ?