IdentityServer 4, Backchannel-Logout, Reject Logout-Token, Bad Request

Question

Situation:

I have my own OpenId based server and several clients. When I log out, I use the backchannel logout to log me out in all clients. The OpenId Specification tells me, that I need to verify the sent logout_token and reject with BadRequest 400 if it cannot be verified.

My implementation in the client (abbreviated to the relevant parts), done in Startup.cs:

public void ConfigureServies(IServiceCollection services)
{
    //... stuff set up
    services.AddAuthentication()
        .AddCookie(//some options)
        .AddOpenIdConnect(OpenIdConnectDefaults.AuthenticationScheme, options =>
        {
            //... stuff set up
            options.Events.OnRemoteSignOut = context =>
            {
                var parameters = context.ProtocolMessage.Parameters;
                if (parameters.TryGetValue("logout_token", out string token))
                {
                    if(...) //check if token is valid according to spec
                    {
                        //do stuff for logout
                    }
                    else
                        return Task.FromResult(new BadRequestResult()); //<--- Here is the core of my question.
                }
                return Task.CompletedTask;
            }
        }
}

My actual Question:

In the marked line: What is the correct syntax to properly give back the Error 400 as required from the Spec? It is possible that I have an important gap in knowledge about Tasks, but I'm not sure what I actually should do here or which keywords to search for.

When I try to test this out with Postman, I get an error 404 back (despite running through the code to that BadRequestResult), instead of the expected 400.

OnRemoteSignOut is defined with a return type of Func<RemoteSignOutContext, Task>.

Official spec for OpenId logout token validation