Bitkub Custom Headers in PostRequest C#

I am making a Post HttpRequest, but although I added Headers to the request message the response I am getting has a field RequestMessage which doesn't contain the Headers I added.

                request.Headers.Add("Accept", "application/json");
                request.Headers.Add("X-BTK-APIKEY", _apiKey._public_key);
                

I am using PostAsync and HttpRequestMessage.Headers.Add to add custom headers to the secure endpoints of Bitkub, but I am getting {"error": 2} (Missing X-BTK-APIKEY) as return. I've tried:

1. Changing PostAsync to SendAsync
2. Changing request.Headers.Add("X-BTK-APIKEY", _apiKey._public_key) to request.Headers.Authorization = AuthenticationHeaderValue("X-BTK-APIKEY", _apiKey._public_key).

But the results are similar.