Most of the APIs now-a-days incorporate oauth 2.0 authentication. It is not as complicated as it may seem at times, provided the right links and documentation are found. Microsoft APIs are extremely helpful and useful, but to access them from a third party application is when its needed, for the entire process of registration and access token retrieval, to be followed; to comply with the oauth authentication in place.

For office365 (2016) APIs the links that would help access these APIs would be of the format https://outlook.office.com/api/{version}/me/…

me – represents the logged in user
{version} – v2.0 or v1.0
/… – events (for outlook calendar API)

The first step, as hundreds of websites mentions, is to register the application. To be a little more comprehensive on this point, I would like to mention that it is not required to deploy any kind of code or application into the registration portal.

https://apps.dev.microsoft.com is free if you have access to a Microsoft office account.

The registration process is a way of letting Microsoft know that a particular app is going to access its APIs. It is a good practice to name the app appropriately as it will appear on the screen when the application, that is being developed, navigates to the login page.

Application id is the client id which is needed to be provided in the headers when requests are made for authorisation code and access token.
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