To test this recipe, you will need a way to easily send HTTP requests. An excellent free tool for this is Postman (https://www.getpostman.com/), which features a nice and self-explanatory UI. If you'd rather not download anything, you can use your terminal for this. If you're on Windows, you can open PowerShell and enter the following to do an HTTP request:
Invoke-WebRequest -UseBasicParsing <Your URL> -Method <Your method in CAPSLOCK> -Body <Your message as a string>
So, if you wanted to POST the message hello there, my echoing friend to http://localhost:3000/echo, as you will be asked to later in the recipe, you'd need to enter the following command:
Invoke-WebRequest -UseBasicParsing http://localhost:3000/echo -Method POST -Body "Hello there, my echoing friend"
On Unix systems, you can use cURL for that (https://curl.haxx.se/). The analog command is the following:
curl -X <Your method> --data <Your message> ...