The 'Authentication Base URI' is shown in the Installed Package in Marketing Cloud. I took that URI, then appended '/v2/token' to it (just for the authorization request) because I'm on a newer version of MC than what is shown in the video. Here's how I get to the 'Authentication Base URI' in my MC instance: Log into Marketing Cloud > hover over my name in the upper right to bring up a settings menu > choose Setup > In the Setup menu on the left choose Apps > choose Installed Packages > Choose the package you created > look under API Integration component that you added to the package to see the 'Authentication Base URI'.