Using form-urlencoded 
https://stackoverflow.com/questions/35325370/how-do-i-post-a-x-www-form-urlencoded-request-using-fetch

```js
const requestBody = new URLSearchParams({
  client_id: clientId,
  client_secret: clientSecret,
  code: code,
  grant_type: "authorization_code",
});

fetch(apiUrl, {
  method: "POST",
  headers: {
    "Content-Type": "application/x-www-form-urlencoded",
  },
  body: requestBody,
})
  .then(response => response.json())
  .then(data => {
    // Handle the response data here
    console.log(data);
  })
  .catch(error => {
    // Handle errors here
    console.error("Error:", error);
  });
```
 
Curl equivalent
```bash
curl -i -X POST "https://api.box.com/oauth2/token" \
     -H "content-type: application/x-www-form-urlencoded" \
     -d "client_id=[CLIENT_ID]" \
     -d "client_secret=[CLIENT_SECRET]" \
     -d "code=[CODE]" \
     -d "grant_type=authorization_code"

```
 
2nd method
```js
var details = {
    'userName': 'test@gmail.com',
    '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
})
```