Why does ASP.NET Core 5 Web API prevent HttpPost request access?

I have configured a Configure method in my Startup.cs file like this:

app.UseCors(option => option
                         .AllowAnyOrigin()
                         .AllowAnyHeader()
                         .AllowAnyMethod() );

But when publishing a project and uploading files to Plesk, the HttpPost method won't work at all, and I get this error:

Cross-Origin Request Blocked: the same origin policy disallows reading the remote resource (Reason: CORS header ‘Access-Control-Allow-Origin’ missing). Status code: 500.

I have been set AllowAnyHeader() previously but it seems it commands does not work. Why - and how can I fix this?

Edit:

I have these snipped in the Web.config:

  <system.webServer>
    <httpProtocol>
      <customHeaders>
        <add name="Access-Control-Allow-Origin" value="*" />
      </customHeaders>
    </httpProtocol>
    <security>
      <requestFiltering>
        <verbs>
          <remove verb="OPTIONS" />
        </verbs>
      </requestFiltering>
    </security>
  </system.webServer>

But whole of site wont run.

Edit:

I have used these snipped in the Startup.cs:

public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllers(); 
            services.AddControllers()
             .AddJsonOptions(o => o.JsonSerializerOptions
                 .ReferenceHandler = ReferenceHandler.Preserve);

            services.AddMemoryCache();

            services.AddCors(opt => opt.AddPolicy("CorsPolicy", c =>
            {
                c.AllowAnyOrigin()
                   .AllowAnyHeader()
                    .AllowAnyMethod();
            }));

        }

        // 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.UseRouting();
            app.UseCors("CorsPolicy");
            app.UseAuthorization();
            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }