Hi @VincentH_NET,
You are right: a native app must not use a secret; it must use the Authorization Code Flow with PKCE.
I would like to suggest that you use our OIDC Client for .NET Desktop SDK, but I don’t know how it is compatible with Uno Platform (sorry, I’m not familiar with Uno). Anyway, you can take a look at its source code to understand how to use IdentityModel.OidcClient (yes, our SDK is a wrapper around this library too)
Basically, you should provide all the OidcClientOptions you reported above but the ClientSecret. Take a look at this code of our SDK.
I hope this helps.