How do I POST a x-www-form-urlencoded request using Fetch?

I have some parameters that I want to POST form-encoded to my server:

{
    'userName': 'test@gmail.com',
    'password': 'Password!',
    'grant_type': 'password'
}

I’m sending my request (currently without parameters) like this

var obj = {
  method: 'POST',
  headers: {
    'Content-Type': 'application/x-www-form-urlencoded; charset=UTF-8',
  },
};
fetch('https://example.com/login', obj)
  .then(function(res) {
    // Do stuff with result
  }); 

How can I include the form-encoded parameters in the request?