Startup.cs

public class Startup {
    private string clientCertPath;
}

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<DataAccess.ClientDatabaseContext>(options =>
        options.UseNpgsql(this.connectionString, opt => {
            opt.RemoteCertificateValidationCallback(RemoteCertificateValidation);
            opt.UseAdminDatabase("defaultdb");
        }));
}

private bool RemoteCertificateValidation(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
    X509Chain chain0 = new X509Chain();
    chain0.ChainPolicy.RevocationMode = X509RevocationMode.NoCheck;
    // add all your extra certificate chain
    chain0.ChainPolicy.ExtraStore.Add(new X509Certificate2(this.clientCertPath));
    chain0.ChainPolicy.VerificationFlags = X509VerificationFlags.AllowUnknownCertificateAuthority;
    return chain0.Build((X509Certificate2)certificate);
}