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

You have to put together the x-www-form-urlencoded payload yourself, like this:

var details = {
    'userName': '[email protected]',
    'password': 'Password!',
    'grant_type': 'password'
};

var formBody = [];
for (var property in details) {
  var encodedKey = encodeURIComponent(property);
  var encodedValue = encodeURIComponent(details[property]);
  formBody.push(encodedKey + "=" + encodedValue);
}
formBody = formBody.join("&");

fetch('https://example.com/login', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/x-www-form-urlencoded;charset=UTF-8'
  },
  body: formBody
})

Note that if you were using fetch in a (sufficiently modern) browser, instead of React Native, you could instead create a URLSearchParams object and use that as the body, since the Fetch Standard states that if the body is a URLSearchParams object then it should be serialised as application/x-www-form-urlencoded. However, you can't do this in React Native because React Native does not implement URLSearchParams.

Even simpler:

fetch('https://example.com/login', {
    method: 'POST',
    body: new URLSearchParams({
        'userName': '[email protected]',
        'password': 'Password!',
        'grant_type': 'password'
    })
});

Docs: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch

Use URLSearchParams

https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams

var data = new URLSearchParams();
data.append('userName', '[email protected]');
data.append('password', 'Password');
data.append('grant_type', 'password');