This is an example of an HTML form:

<form method="POST" action="/submit-form">
  <input type="text" name="username" />
  <input type="submit" />
</form>

When the user press the submit button, the browser will automatically make a POST request to the /submit-form URL on the same origin of the page, sending the data it contains, encoded as application/x-www-form-urlencoded. In this case, the form data contains the username input field value.

Forms can also send data using the GET method, but the vast majority of the forms you’ll build will use POST.

The form data will be sent in the POST request body.

To extract it, you will use the express.urlencoded() middleware, provided by Express:

const express = require('express')
const app = express()

app.use(express.urlencoded())

Now you need to create a POST endpoint on the /submit-form route, and any data will be available on Request.body:

app.post('/submit-form', (req, res) => {
  const username = req.body.username
  //...
  res.end()
})

Don’t forget to validate the data before using it, using express-validator.