Blazor WASM NetworkError when attempting to fetch resource. "CORS Missing Allow Origin"

In this Blazor WASM application I am simply trying to consume a third party API - I open up developer tools and see this message "CORS Missing Allow Origin" when I attempt to retrieve the data. In this case I did add request.Headers.Add("Access-Control-Allow-Origin", "*"); to the request but it is still failing. I have verified this result in different browsers other than FF.

The code for the Index.Razor page is here:

@page "/"  @inject HttpClient Http  <h3>Presentations Scheduled</h3>    <form>      <fieldset class="form-group">          <div>              <button type="button" class="btn btn-dark mr-sm-2 mb-2"                      @onclick=@(async ()=>  await GetToday())>                  Today              </button>          </div>      </fieldset>        @if (string.IsNullOrWhiteSpace(errorString) == true)      {          <div class="h2">No Data</div>      }      else if (string.IsNullOrWhiteSpace(errorString) == false)      {          <div class="h2">@errorString</div>      }  </form>    <div class="h2">@TestReturn</div>          @code  {      TestModel TestReturn;      string errorString = null;      string TestBaseURL;      HttpRequestMessage request;        private async Task GetToday()      {          var byteArray = Encoding.ASCII.GetBytes("user:pass");          Http.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", Convert.ToBase64String(byteArray));            string todayDate = DateTime.Today.AddDays(0).ToString("yyyyMMdd");          TestBaseURL = "https://webservices.xcompany.com/";          UriBuilder TestURI = new UriBuilder(TestBaseURL);          TestURI.Scheme = "https";          TestURI.Path = "ws/run/reservations.json";          TestURI.Query = "resource_query_id=246492";          var TestQuery = HttpUtility.ParseQueryString(TestURI.Query);          TestQuery["scope"] = "extended";          TestQuery["start_dt"] = todayDate;          TestQuery["end_dt"] = todayDate;          TestURI.Query = TestQuery.ToString();          TestBaseURL = TestURI.ToString();          request = new HttpRequestMessage(HttpMethod.Get, TestBaseURL);          request.Headers.Add("Access-Control-Allow-Origin", "*");            try          {              using var httpResponse = await Http.SendAsync(request);              TestReturn = await httpResponse.Content.ReadFromJsonAsync<TestModel>();          }          catch (Exception ex)          {              errorString = $"There was a problem: {ex.Message}";          }        }    }  

https://stackoverflow.com/questions/65401899/blazor-wasm-networkerror-when-attempting-to-fetch-resource-cors-missing-allow December 22, 2020 at 08:48AM