After creating an Installed Package under Setup > Apps > Installed Package, using the Authentication Base URI is needed to retrieve the Bearer Auth token. This token is needed to run any API endpoints.

The endpoint for the Authentication looks something like this:

https://uniqueprefixhere.auth.marketingcloudapis.com

Note that the [uniqueprefixhere] is a garbled mess of alphanumeric letters and numbers that is unique to the package that is created.

The specific endpoint for the V2 token looks like this:

https://uniqueprefixhere.auth.marketingcloudapis.com/v2/token/?

You can use Postman to POST to the endpoint using the grant_type of “client_credentials” and also include your client_id and client_secret that was provided when the Installed App was created. The Postman screenshot looks like this:

Note that the sensitive information like the Authentication URI prefix, client_id, client_secret, and access_token are hidden for privacy and security reasons.

Copy and paste that access_token value and it can be used in the “Authorization” part of Postman when calling other API endpoints.

More details on Using Postman to Test API Access Token Request