So POST parameters arrive in the HTTP request body, and this is handled as a stream of data chunks by node.js. So the first thing you must do is make sure you assemble the stream of chunks into a complete piece of data. Then you may want to parse it as either url encoded or JSON if that's what it is. The standard middleware for this is body-parser. You set it up like they say in the README:
const express = require('express')
const bodyParser = require('body-parser')
const app = express()
// parse application/x-www-form-urlencoded
app.use(bodyParser.urlencoded({ extended: false }))
// parse application/json
app.use(bodyParser.json())
// parse application/vnd.api+json as json
app.use(bodyParser.json({ type: 'application/vnd.api+json' }))
app.use(function (req, res, next) {
console.log(req.body) // populated!
next()
})