Read Payload data from scratch as seen in https://www.stevejgordon.co.uk/using-the-bodyreader-and-bodywriter-in-asp-net-core-3-0

ReadPostData.cs

using System;
using System.Buffers;
using System.IO.Pipelines;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System.Text.Json;

namespace WebApplicationTestUrl
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            // services.AddControllers();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseHttpsRedirection();

            //app.UseRouting();

            //app.UseAuthorization();

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


            app.UseRouting();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapPost("/api/device", async context =>
                {
                    var cancellationToken = context.RequestAborted;
                    var model = await ReadModelAsync(context.Request.BodyReader, cancellationToken);

                    await context.Response.WriteAsync("Hello World!");
                });
            });
        }

        private static async Task<DeviceModel> ReadModelAsync(PipeReader reader, CancellationToken cancellationToken)
        {
            DeviceModel model = null;

            while (!cancellationToken.IsCancellationRequested)
            {
                var readResult = await reader.ReadAsync(cancellationToken);
                var buffer = readResult.Buffer;

                var position = buffer.PositionOf((byte)'}');

                if (position != null)
                {
                    if (buffer.IsSingleSegment)
                    {
                        model = JsonSerializer.Deserialize<DeviceModel>(buffer.FirstSpan, new JsonSerializerOptions
                        {
                            PropertyNameCaseInsensitive = true
                        });
                    }
                    else
                    {
                        using var document = JsonDocument.Parse(buffer);

                        if (document.RootElement.TryGetProperty("StationId", out var pathProperty)
                            && pathProperty.GetType() == typeof(string))
                        {
                            model = new DeviceModel
                            {
                                StationId = new Guid(pathProperty.GetString())
                            };
                        }
                    }
                }

                reader.AdvanceTo(buffer.Start, buffer.End);

                if (readResult.IsCompleted) break;
            }

            return model;
        }
    }

    public class DeviceModel
    {
        public Guid StationId { get; set; }
    }
}