HTTP POST multipart form-data

Question

I'm working on an App in Xamarin.Forms. I want to send the Post Request to an API with form-data. In the code below, it returns the success message, but the data isn't posted there.

public class Post {
    public string ConnectionId { get; set; }
    public string HolderFirstName { get; set; }
    }

public async void SendProof(object sender, EventArgs e) {
    try {
    Uri URL = new Uri(string.Format("http://11.222.333.44:4000/api/v1/Proof/SendProofNameRequest"));
    HttpClient _client = new HttpClient();

    var post = new Post {ConnectionId = "9c12dba2-6cb9-4382-8c96-f1708a7e8816", HolderFirstName = "Carl Dreyer"};
    var content = JsonConvert.SerializeObject(post);
    var response = await _client.PostAsync(URL, new StringContent(content, Encoding.UTF8, "multipart/form-data"));

    if (response.IsSuccessStatusCode) { 
    await DialogService.AlertAsync("Credential Proof Sent Successfully!");}
    }

    catch(Exception error) {
    await DialogService.AlertAsync(error.Message); }
    }

Binding for Button which triggers this Function.

public ICommand SendProofCommand => new Command(() => SendProof(default, default));

Answer 1

public async Task SendProof()
        {
            try {
                HttpClient client = new HttpClient(new NativeMessageHandler());
                client.BaseAddress = new Uri("http://11.222.333.44:4000/");
                var postData = new List<KeyValuePair<string, string>>();

                var nvc = new List<KeyValuePair<string, string>>();

                nvc.Add(new KeyValuePair<string, string>("ConnectionId", "9c12dba2-6cb9-4382-8c96-f1708a7e8816"));
                nvc.Add(new KeyValuePair<string, string>("HolderFirstName", "Bergman"));
                var req = new HttpRequestMessage(HttpMethod.Post, "http://11.222.333.44:4000/" + "api/v1/Proof/SendProofNameRequest") { Content = new FormUrlEncodedContent(nvc) };

                var res = await client.SendAsync(req);

                if (res.IsSuccessStatusCode)
                {
                    await DialogService.AlertAsync("Proof Sent Successfully!");
                }
                else
                {
                    await DialogService.AlertAsync("Unsuccessfull!");
                }
            }
            catch(Exception error) {
            await DialogService.AlertAsync(error.Message);
            }
        }