Example configuring SameSite Cookie Policy in ASP.NET Core 3.1.

// ------------------------------------------------------------------------------------------------------------
// File: SameSiteHandlingExtensions.cs
// Source: https://github.com/IdentityServer/IdentityServer4.Demo/blob/main/src/IdentityServer4Demo/SameSiteHandlingExtensions.cs
// ------------------------------------------------------------------------------------------------------------

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;

namespace IdentityServer4Demo
{
    // copied from https://devblogs.microsoft.com/aspnet/upcoming-samesite-cookie-changes-in-asp-net-and-asp-net-core/
    public static class SameSiteHandlingExtensions
    {
        public static IServiceCollection AddSameSiteCookiePolicy(this IServiceCollection services)
        {
            services.Configure<CookiePolicyOptions>(options =>
            {
                options.MinimumSameSitePolicy = SameSiteMode.Unspecified;
                options.OnAppendCookie = cookieContext => CheckSameSite(cookieContext.Context, cookieContext.CookieOptions);
                options.OnDeleteCookie = cookieContext => CheckSameSite(cookieContext.Context, cookieContext.CookieOptions);
            });

            return services;
        }

        private static void CheckSameSite(HttpContext httpContext, CookieOptions options)
        {
            if (options.SameSite == SameSiteMode.None)
            {
                var userAgent = httpContext.Request.Headers["User-Agent"].ToString();
                if (DisallowsSameSiteNone(userAgent))
                {
                    // For .NET Core < 3.1 set SameSite = (SameSiteMode)(-1)
                    options.SameSite = SameSiteMode.Unspecified;
                }
            }
        }

        private static bool DisallowsSameSiteNone(string userAgent)
        {
            // Cover all iOS based browsers here. This includes:
            // - Safari on iOS 12 for iPhone, iPod Touch, iPad
            // - WkWebview on iOS 12 for iPhone, iPod Touch, iPad
            // - Chrome on iOS 12 for iPhone, iPod Touch, iPad
            // All of which are broken by SameSite=None, because they use the iOS networking stack
            if (userAgent.Contains("CPU iPhone OS 12") || userAgent.Contains("iPad; CPU OS 12"))
            {
                return true;
            }

            // Cover Mac OS X based browsers that use the Mac OS networking stack. This includes:
            // - Safari on Mac OS X.
            // This does not include:
            // - Chrome on Mac OS X
            // Because they do not use the Mac OS networking stack.
            if (userAgent.Contains("Macintosh; Intel Mac OS X 10_14") &&
                userAgent.Contains("Version/") && userAgent.Contains("Safari"))
            {
                return true;
            }

            // Cover Chrome 50-69, because some versions are broken by SameSite=None,
            // and none in this range require it.
            // Note: this covers some pre-Chromium Edge versions,
            // but pre-Chromium Edge does not require SameSite=None.
            if (userAgent.Contains("Chrome/5") || userAgent.Contains("Chrome/6"))
            {
                return true;
            }

            return false;
        }
    }
}

// ------------------------------------------------------------------------------------------------------------
// File: Startup.cs
// Source: https://github.com/IdentityServer/IdentityServer4.Demo/blob/main/src/IdentityServer4Demo/Startup.cs
// ------------------------------------------------------------------------------------------------------------

using Microsoft.AspNetCore.Builder;
using Microsoft.Extensions.Configuration;

namespace IdentityServer4Demo
{
    public class Startup
    {
        public IConfiguration Configuration { get; }

        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public void ConfigureServices(IServiceCollection services)
        {
            // cookie policy to deal with temporary browser incompatibilities
            services.AddSameSiteCookiePolicy();
        }

        public void Configure(IApplicationBuilder app)
        {
            app.UseCookiePolicy();
        }
    }
}