IdentityServer - client definition in appsettings.json

I tried to define my on client_id in IdentityServerConfig section of appsettings.json file. E.g. goes here:

But when I try to call /connect/token API with client_id=GMG2 I get an error that no such client_id exists.

Is there any possibility to define my own client, client_id and client_secret in appsettings.json? Thank you for your help.