Last year I posted on using Postman. Things have changed since then and I need to update. http://dynamicsnavax.blogspot.com/2017/05/dynamics-365-for-operation-web-service.html There is a good article that Microsoft has written which I followed without any issues. https://docs.microsoft.com/en-us/dynamics365/unified-operations/dev-itpro/data-entities/third-party-service-test#prerequisites Below are some screenshot incase you are a visual person like me. In the environment set ups. It should looks something like this. When you run it, you should get a response. Once you got the token. You are good to go with your messages. Below is the same example from the blog post. You run into any problems, click on the console icon at the bottom. Should give you a bit more information. If you get a 401 error. It is usually a typo. Make sure you got the spaces and backslashes correct. One simple character can drive you crazy.