Providing a custom 401 response message on OpenId OnRedirectToIdentityProvider

I'm using AddOpenIdConnect and need to modify the response in case the OnRedirectToIdentityProvider event is raised. Within this event the response status is modified to 401 and I would like to set a custom message. To write this custom message, I've created the SetResponseBody method.

The solution of this post is used to set the response status, but I need to modify the Body as well.

I'm calling the SetResponseBody method (a custom method which I implemented) in order to modify the response body as soon as the OnRedirectToIdentityProvider event is raised from AddOpenIdConnect.'

As mentioned in one of the comments by @Panagiotis Kanavos in the post, the SetResponseBody method doesn't seem to be a correct solution (despite the response actually contains valid json). Could you provide an alternative?

Summarized: I would like to return a custom response besides the status code 401.

OnRedirectToIdentityProvider = async e =>
{
    // e is of type RedirectContext
    if (e.Request.Path.StartsWithSegments("/api")))
    {            
        if (e.Response.StatusCode == (int)HttpStatusCode.OK)
        {
            e.Response.StatusCode = (int)HttpStatusCode.Unauthorized;
            // TestMessage is a const
            // e.Response is readonly (get) so it's not possible to set it directly.
            await ResponseRewriter.SetResponseBody(e.Response, TestMessage);
        }
        e.HandleResponse();
    }
    await Task.CompletedTask;
}

with ResponseRewriter.SetResponseBody defined as follows

public static async Task SetResponseBody(HttpResponse response, string message)
{   
    var originalResponseBody = response.Body;
    var responseBody = new MemoryStream();
    response.Body = responseBody;
    response.ContentType = "application/json";
    var body = new { Message = message };
    string json = JsonSerializer.Serialize(body);
    await response.WriteAsync(json);
    response.Body.Seek(0, SeekOrigin.Begin);
    await responseBody.CopyToAsync(originalResponseBody);         
}

The last two lines are implemented as written in this post.