When using the .NET Framework, you must include the Microsoft.Windows.SDK.Contracts nuget package and then you will have access to (some) WinRT APIs.

Here is a sample C# Console Application:

class Program
{
    static async Task Main()
    {
        await Windows.Security.Credentials.UI.UserConsentVerifier.RequestVerificationAsync("hello world").AsTask();
    }
}

It works because this API is marked as DualApiPartition.

Also make sure you're using PackagesReference format for nuget packages and not the packages.config configuration.