It says that for the example purpose they will use the in memory store:
For this document we will use the in-memory version of the client store. You can wire up the in-memory store in
ConfigureServices
via theAddInMemoryClients
extensions method.
I am not sure if this is mandatory for using external clients or not and searching online didn't yield any definitive answer.
I found however this in the Microsoft documentation: Use Identity to secure a Web API backend for SPAs | Microsoft Learn
It shows that there are predefined IdentityServer profiles (whatever that means). Not sure if this helps in any way though.
Lastly a lot of .NET services support the services.Configure
method for further configuration. E.g. in the OnConfigureServices partial method you can try:
services.Configure<ApiAuthorizationOptions>(options => options.Clients.Add(...))