There are a lot of ways to call D365 web services – Postman, Fiddler, Logic Apps – but they're all a bit fussy. Getting authentication right the first time can be stressful, and they all require additional downloads, accounts, or subscriptions.
Here is a quick example project I made. It's a simple x++ class with a few test methods. One is a simple Hello World, and the other, HelloParameter, will take 2 parameters, integers, and return the sum of them with a friendly message.
You can already call a service without parameters simply through URL navigation, so the real value is being able to pass parameters. To call the service's HelloParameter method, we can write a quick snippet in Chrome (F12 shortcut).
The snippet sets two parameters, integers with values 1 and 4 respectively. A POST request is made, and it returns a sum of 5.