To find your organization's My Domain name, go to Setup >> Quick Find: My Domain (see below).

If you are installing into a sandbox, then you will need the name of the sandbox as well.  To find your sandbox name, (while logged into the sandbox) look at the top of any page (see below).

Based on how you originally set up the SharePoint application registration for authentication within your organization, you may proceed with any of the options provided below:

1. If authentication is done using the SharePoint Add-in:
   • Please follow this article and create a new web application.
2. If authentication is done using Azure-AD:
   • Log in to your Azure portal as an administrator.  Then, under Azure Services, select Azure Active Directory.
   • On the Active Directory page, select App registrations from the navigation menu on the left.
   • Find and open your application by clicking on the name.
   • From the navigation menu on the left, select Authentication.
   • Under the Web Redirect URIs, click on Add URI action link.
   • Enter your assembled Redirect URL in the input box indicated on your screen.
   • Click Save to save your changes to the application.

You can find SharePoint Setup in the AppLauncher (waffle) at the top left of any Salesforce page.

1. From the side menu, click on SharePoint Configuration.
2. Select the Edit action button in the page header.
3. Enter your assembled Redirect URL.
4. Click Save.