POST /api/2.0/authentication.json HTTP/1.1
Host: portal.onlyoffice.com
Accept: application/json,application/xml
Accept-Encoding: gzip, deflate
userName=yourusername&password=yourpassword

HTTP/1.1 200 Ok
Cache-Control: private
Content-Type: application/json; charset=utf-8
{
    "count": 0,
    "startIndex": 0,
    "status": 0,
    "response": {
    "token": "sdjhfskjdhkqy739459234",
    "expires": "2013-01-13T16:35:42.7564317+04:00"
    }
}

private async Task<string> Authorize(string userName, string password)
{
     var uri = new Uri("http://portal.onlyoffice.com/api/2.0/authentication");
     var formContent = new FormUrlEncodedContent(new[]
     {
            new KeyValuePair<string, string>("userName", userName),
            new KeyValuePair<string, string>("password", password)
     });

     var client = new HttpClient();
     var response = await client.PostAsync(uri.ToString(), formContent);
     var responseContent = await response.Content.ReadAsStringAsync();
     dynamic jsonResult = JObject.Parse(responseContent);
     return jsonResult.token;
}

public async Task<ActionResult> LoginInExternal()
{
    var token = await Authorize("email@gmail.com", "password");
    return View();
}