Calling OCRSDK by Abbyy from UWP. This is how you do it.

Abbyy.UWP.cs

// create http cache
if (_cachedClient is null)
{
    // create credntial for http client
    var credentials = new Windows.Security.Credentials.PasswordCredential
    {
        UserName = _settingsService.AbbyyApplicationId,
        Password = _settingsService.AbbyyPassword,
    };
    var handler = new HttpBaseProtocolFilter { ServerCredential = credentials, };
    _cachedClient = new Windows.Web.Http.HttpClient(handler);
}

// set http client
var client = _cachedClient;

// call to process image
// read file as stream
var stream = await image.OpenStreamForReadAsync();
var file = new HttpStreamContent(stream.AsInputStream());

// build process image url
var url = $"{_settingsService.AbbyyServiceUrl}/{_processImageRelative}";
if (!Uri.TryCreate(url, UriKind.Absolute, out var uri))
{
    throw new InvalidCastException($"OcrEndpoint is an invalid Uri: {url}");
}

// call the service
var response = await client.PostAsync(uri, file);
response.EnsureSuccessStatusCode();
var json = await response.Content.ReadAsStringAsync();
var result = ParseJson(json);

// keep going until completed
while (!Equals(result.Status, "Completed"))
{
    // wait for a time
    await Task.Delay(1000);

    // build get status url
    url = $"{_settingsService.AbbyyServiceUrl}/{_taskStatusRelative}{result.TaskId}";
    if (!Uri.TryCreate(url, UriKind.Absolute, out uri))
    {
        throw new InvalidCastException($"TaskStatus is an invalid Uri: {url}");
    }

    // call for status
    var text = await client.GetStringAsync(uri);
    result = ParseJson(text);
}

if (!Uri.TryCreate(result.Url, UriKind.Absolute, out uri))
{
    throw new InvalidCastException($"Blob is an invalid Uri: {url}");
}

// download results
var xml = await client.GetStringAsync(uri);

// convert to object
var doc = new XmlDocument();
doc.LoadXml(xml);

// read all charParams
var chars = doc.GetElementsByTagName("charParams");
var elements = chars.OfType<XmlElement>();

// convert to letters
var letters = elements.Select(x => new 
{
    Rectangle = new System.Drawing.Rectangle(
            int.Parse(x.Attributes["l"].Value),
            int.Parse(x.Attributes["t"].Value),
            int.Parse(x.Attributes["r"].Value) - int.Parse(x.Attributes["l"].Value),
            int.Parse(x.Attributes["b"].Value) - int.Parse(x.Attributes["t"].Value)),
    Text = x.InnerText,
});