using (HttpClient httpClient = new HttpClient())
{

    string trustPilotAccessTokenUrl = "https://api.trustpilot.com/v1/oauth/oauth-business-users-for-applications/accesstoken";

    httpClient.BaseAddress = new Uri(trustPilotAccessTokenUrl);

    httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/x-www-form-urlencoded"));

    var authString = "MyApiKey:MyApiSecret";

    httpClient.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", Base64Encode(authString));

    var stringPayload = "grant_type=password&username=MyUserEmail&password=MyPassword";

    var httpContent = new StringContent(stringPayload, Encoding.UTF8, "application/x-www-form-urlencoded");

    HttpResponseMessage httpResponseMessage = httpClient.PostAsync(trustPilotAccessTokenUrl, httpContent).Result;

    var accessTokenResponseString = httpResponseMessage.Content.ReadAsStringAsync().Result;

    var accessTokenResponseObject = JsonConvert.DeserializeObject<AccessTokenResponse>(accessTokenResponseString);


    // Create invitation object
    var invitation = new ReviewInvitation
    {
        ReferenceID = "inv001",
        RecipientName = "Jon Doe",
        RecipientEmail = "Jon.Doe@comp.com",
        Locale = "en-US"
    };

    var jsonInvitation = JsonConvert.SerializeObject(invitation);

    var client = new HttpClient();
    client.DefaultRequestHeaders.Add("token", accessTokenResponseObject.AccessToken);

    var invitationsUri = new Uri("https://invitations-api.trustpilot.com/v1/private/business-units/4be7c5b0000064000509d532/invitations");

    // This here as a status of WaitingForActivation!
    var a = client.PostAsync(invitationsUri, new StringContent(jsonInvitation)).ContinueWith((postTask) => postTask.Result.EnsureSuccessStatusCode());

}