How to send form data via a POST request.

URL-encoded form body (simple fields)

For plain key/value data, use application/x-www-form-urlencoded. URLSearchParams builds the body and encodes each value for you:

const body = new URLSearchParams();
body.set('username', 'alice');
body.set('message', 'Hello, World!');

fetch('https://api.example.com/submit', {
  method: 'POST',
  headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
  body,   // URLSearchParams sets the encoding; fetch sends "username=alice&message=Hello%2C+World%21"
});

You don’t set the header manually when passing URLSearchParams to fetch in the browser — but setting it explicitly (as above) is harmless and clear.

File uploads: multipart/form-data

When you’re sending files, use FormData, which produces multipart/form-data. Let the browser set the Content-Type (it adds the required boundary) — do not set it yourself:

const form = new FormData();
form.append('username', 'alice');
form.append('avatar', fileInput.files[0]);   // a File object

fetch('https://api.example.com/upload', {
  method: 'POST',
  body: form,   // no Content-Type header — the browser adds multipart/form-data; boundary=...
});

Which format?

Python

With requests, pass data= for url-encoded, files= for multipart:

import requests
# url-encoded
requests.post(url, data={'username': 'alice', 'message': 'Hello, World!'})
# multipart (file upload)
requests.post(url, files={'avatar': open('a.png','rb')}, data={'username': 'alice'})

cURL

# url-encoded — --data-urlencode encodes each field
curl -X POST https://api.example.com/submit \
  --data-urlencode "username=alice" \
  --data-urlencode "message=Hello, World!"

# multipart (file)
curl -X POST https://api.example.com/upload \
  -F "username=alice" -F "avatar=@a.png"

A note on the standards

The x-www-form-urlencoded format is defined by the WHATWG URL Standard (a space becomes +); multipart/form-data is defined by RFC 7578. They’re different formats for different jobs, not interchangeable.

The form encoder builds a url-encoded body from key/value pairs and gives you a matching cURL command to test with.