Introduction

There are lots of ways to access the SharePoint API to fetch or update its resources. In each of the ways, authentication plays an important role in in authorizing access to get the information. As a developer, you may have used the PostMan tool for accessing the REST APIs.
Postman Chrome Extension

This is a developer friendly tool for handling the REST APIs from any platform. By using this tool we’ll fetch or update any information from SharePoint using REST API endpoints.
Example
Let’s take a simple example like, getting the web title from the current site context. The syntax for retrieving the website’s title is
https://<SiteName>.sharepoint.com/_api/web?$select=Title
After entering the above URL in the text-box, we will get the Unauthorized exception on accessing the information. Because SharePoint Online is very secured and doesn’t allow anonymous users to access the information for their site. The below is the error message response, after sending the request.
Fig 1: UnAuthorized from Postman
To avoid the Unauthorized exception, we have to add some request header values to the API request.
Authentication Policies
SharePoint online considers any one of the below three types of polices to authenticate the Add-In.
Request Headers
And, we require the following information in various requests to authenticate with SharePoint online site.

Authorize Postman to access SharePoint

To get authorized from external system, we should pass access-token value as a request header along with the REST API URL. Before that we have to get the access-token, for that we should generate Client Id and Secret information from the site by registering as an App Only Add-In in SharePoint site. This is same as like registering add-in for Provider Hosted Add-In.
I have provided the steps below to get the Tenant Id, Access Token and data from SharePoint using PostMan utility.
Register Add-In
On the initial stage, we must register the Add-In in SharePoint, where we want to access the information. Follow the steps below to register the Add-In in the SharePoint site.
Grant Permissions to Add-In
Once the Add-In is registered, we have to set the permissions for that add-in to access the SharePoint data. We will set the Read permission level to the web scope, so that we will be able to read the web information.
Note: If we want to access site collection or tenant level, we have added the xml accordingly
Retrieve the Tenant ID
Once we have registered the Client Id and Secret with the permissions, we are ready to access the SharePoint information from external system or tools.
At first, we have to know the Tenant ID. Follow the below steps to obtain that information from postman. Postman helps to get the tenant Id by requesting the below url with Authorization header.
Generate the Access Token
In response header, we will get WWW-Authenticate as one of the headers and that contains the necessary information required for the next step. The realm value contains the tenant id for the SharePoint Online site and clientid value contains the resource information (we’ll use it later).
Once we have received the access token, we get the authorization to access the SharePoint data based on the permission applied in Grant Permission as Add-In section.
We have to pass the access token as “token_type access_token”.
Access the SharePoint resource
Now we have the access token, so we can now pass this token in Authorization header with the SharePoint REST API to get the information.

Conclusion

That concludes our exploration of how the Postman utility helps us to test the REST API endpoint before starting development. In the she same wa, we can retrieve or update any information from SharePoint supported by SharePoint REST API endpoints.