pipeline.cs

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.Use(async (context, next) =>
    {
        await next.Invoke();

        if (context.Response.StatusCode == StatusCodes.Status401Unauthorized)
        {
            var customResponse = new { Code = "401", CustomMessage = "Maybe try logging in first!" };
            await context.Response.WriteAsync(JsonConvert.SerializeObject(customResponse));
        }
    });

    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }

    app.UseHttpsRedirection();

    app.UseRouting();

    app.UseAuthorization();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllers();
    });
}
}