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.  

You can run snippets of JavaScript on any page with Chrome.  Javascript can make JSON POST requests, and fortunately for us, "any page" includes the D365 default dashboard homepage.

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.