HttpClient Post data form-url-encoded (national encoding - win-1250)

Question

I am trying post data with HttpClient. My code is like this:

  public static string[] SendData(string adress, Dictionary<string, string> data)
    {
        Task<string> komunikace = PostResponseString(adress, new FormUrlEncodedContent(data));
        var vysledek = komunikace.Result;
        return vysledek.ToString().Split('\n');
    }
  static async Task<string> PostResponseString(string uri, FormUrlEncodedContent data)
    {
        var response = await httpClient.PostAsync(uri, data);
        var contents = await response.Content.ReadAsStringAsync();
        return contents;
    }

I want to send data.Value in windows-1250 encoding, cause site is written in this encoding.

Example: data.Key = apolozka
         data.Value = příbalový leták 822
result: apoznamka=p%F8%EDbalov%FD+let%E1k+822

Don't now, how to do this. Sorry for my poor english.

EDIT:

This is my solution based on: HttpClient FormUrlEncodedContent Encoding Thank you Christoph Lütjen for your comment.

static async Task<string> PostResponseString(string uri, string postData)
        {
            string element = "";
            string elementHttp;
            int poziceAnd;
            for (int i = 0; i < postData.Length; i++)
            {
                if (postData[i] == '=')
                {
                    if (postData.Substring(i + 1).Contains('&'))
                    {
                        poziceAnd = postData.IndexOf('&', i);
                        element = postData.Substring(i + 1, poziceAnd - i - 1);
                    }
                    else
                    {
                        element = postData.Substring(i + 1);
                    }
                    elementHttp = HttpUtility.UrlEncode(element, Encoding.GetEncoding(myEncoding));
                    postData = postData.Replace(element, elementHttp);
                }
            }
            byte[] bytes = System.Text.Encoding.GetEncoding(myEncoding).GetBytes(postData);
            ByteArrayContent content = new ByteArrayContent(bytes);
            content.Headers.Add("Content-Type", "application/x-www-form-urlencoded");
            var response = await httpClient.PostAsync(uri, content);
            byte[] bytesResp = await response.Content.ReadAsByteArrayAsync();
            string odpoved = Encoding.GetEncoding(myEncoding).GetString(bytesResp, 0, bytesResp.Length);
            return odpoved;
        }