Exampple of application/x-www-form-urlencoded string
CorrelationId=1&PickedNumbers%5B%5D=1&PickedNumbers%5B%5D=2&PickedNumbers%5B%5D=3&Picked
You can use qs if you're using node, or browserify.
var qs = require('qs')
var encodedString = "CorrelationId=1&PickedNumbers%5B%5D=1&PickedNumbers%5B%5D=2&PickedNumbers%5B%5D=3&PickedNumbers%5B%5D=4"
console.log(qs.parse(encodedString))
// { CorrelationId: '1', PickedNumbers: [ '1', '2', '3', '4' ] }