Server-side Blazor Post using HttpClient with x-www-form-urlencoded

I am able to use Postman to make a successful Post a request with content type x-www-form-urlencoded to a REST API.

However, when I try to make the same post in a Blazor Server-Side app, I get back a 401 error.

The relevant part of my code is as follows:

FormUrlEncodedContent content = new(new Dictionary<string, string>()
            {
                ["qualification_id"] = qualification.QualificationId,
                ["period_id"] = qualification.PeriodId,
                ["taken_date"] = qualification.CompletedDateString,
                ["expiration_date"] = qualification.ExpirationDateString,
            });
            content.Headers.ContentType = new MediaTypeHeaderValue("application/x-www-form-urlencoded");

            var request = new HttpRequestMessage(HttpMethod.Post, requestUri);
            request.Headers.Add("x-api-key", xApiKey);

            var client = _clientFactory.CreateClient();

            var response = await client.PostAsync(requestUri, content);

            var ret = await response.Content.ReadFromJsonAsync<MemberQualification>();

            return ret;

Any suggestions would be greatly appreciated. NB - the API seems to return a 401 error for just about anything including, I think errors that should be 400 - and I am confident that my x-api-key credentials are correct and have been used successfully elsewhere in other httpClient calls.