C# problems with configure cors

I have been trying for several hours over the last few days to fix my cors problems. My JS is running in the brower to call my currently locally running endpoint.
I am having Cors problems and I have tried several things to get on with my local development.
I have read in some articles and Stackoverflow post that chrome can not handling cors ob localhost so I have also tried this.

open -a Google\ Chrome --args --disable-web-security --user-data-dir

But it did not work. Also with other browsers.

Here is my C# Code.

using Amazon;
using Amazon.DynamoDBv2;
using Customers.Api.Contracts.Responses;
using Customers.Api.Repositories;
using Customers.Api.Services;
using Customers.Api.Validation;
using FastEndpoints;
using FastEndpoints.Swagger;

var builder = WebApplication.CreateBuilder(args);
var config = builder.Configuration;
var  MyAllowSpecificOrigins = "_myAllowSpecificOrigins";
builder.Services.AddCors(options =>
{
    options.AddPolicy(name: MyAllowSpecificOrigins,
        builder =>
        {
            builder.WithOrigins("127.0.0.1:5000","127.0.0.1:5001", "http://127.0.0.1:5000","http://localhost:5000"
                "http://127.0.0.1:5001").AllowAnyMethod().AllowAnyHeader();
        });
});

builder.Services.AddAWSLambdaHosting(LambdaEventSource.HttpApi);
builder.Services.AddFastEndpoints();
builder.Services.AddSwaggerDoc();
builder.Services.AddSingleton<IAmazonDynamoDB>(_ => new AmazonDynamoDBClient(RegionEndpoint.EUCentral1));
builder.Services.AddSingleton<ICustomerRepository>(provider =>
    new CustomerRepository(provider.GetRequiredService<IAmazonDynamoDB>(),
        config.GetValue<string>("Database:TableName")));
builder.Services.AddSingleton<ICustomerService, CustomerService>();
var app = builder.Build();
app.UseCors(MyAllowSpecificOrigins);
app.UseMiddleware<ValidationExceptionMiddleware>();
app.UseFastEndpoints(x =>
{
    x.ErrorResponseBuilder = (failures, _) =>
    {
        return new ValidationFailureResponse
        {
            Errors = failures.Select(y => y.ErrorMessage).ToList()
        };
    };
});
app.UseOpenApi();
app.UseSwaggerUi3(s => s.ConfigureDefaults());
app.Run();

and this is my Javascript code

var payload = {
              event: eventData,
              signupType: signupType,
              name: name,
              email: email,
              birthdate: birthdate,
              gender: gender,
              orientation: orientation,
              partner: {
                name: partnerName,
                email: partnerEmail,
                birthdate: partnerBirthdate,
                gender: partnerGender,
                orientation: partnerOrientation,
              }
            };
          
            
            fetch('http://127.0.0.1:5000/customers', {
              method: 'POST',
              headers: {
                "Content-Type": "application/json"
              },
              body: JSON.stringify(payload)
            })
            .then(function(response) {
              if (!response.ok) {
                throw new Error('Network response was not ok');
              }
              return response.json();
            })
            .then(function(data) {
              console.log('Success:', data);
            })
            .catch(function(error) {
              console.error('Error:', error);
            });
          });
          

This are my error messages...

I haven't any idea what I can try now. And I think about to rewrite this piece in node.js and express because there I haven't normally problems with configure the cors rules...

Based on the error from your browser it looks like the origin is 'null'. So, what I would suggest you try is adding the string "null" to the list of origins when you add the .WithOrigins parameter.

The problem is that you've added the IP/Ports of the request destination in there (your .NET App) and not the origin of the request which in your case is not being found and is being set to null.

Can I ask what the address is of the site you're calling your Javascript from? Also, does it work when you add the string "*" to the list of origins? Keep in mind you don't want this in a production level environment setting.