JAVASCRIPT

use application/x-www-form-urlencoded in javascript

fetch('https://example.com/login', {
    method: 'POST',
    headers:{
      'Content-Type': 'application/x-www-form-urlencoded'
    },    
    body: new URLSearchParams({
        'userName': 'test@gmail.com',
        'password': 'Password!',
        'grant_type': 'password'
    })
});