Web API POST method

While not mandatory, you should follow good practice and encapsulate the payload into a model.

public class MyModel {
    public sting word { get; set; }
    public sting userId { get; set; }
}

You can then use the FromBody parameter attribute to bind the model to the data sent.

Finally addressing the resource not found issue, ensure that the controller is following the proper convention given that the OP is configured with convention-based routing.

public class IdentificationController : ApiController {

    [HttpPost]
    public Probabilities Post([FromBody] MyModel model) {
        string word = model.word;
        string userId = model.userId;
        Request request = new Request();
        request.Identify(word, userId);
        Probabilities probabilities = probabilitiesFactory.GetBy(request.ProbabilitiesId, "Id");
        return probabilities;
    }
}

Reference: Parameter Binding in ASP.NET Web API

Reference: Routing in ASP.NET Web API

There are 2 different solutions for this problem. First: Send word and userId as QueryStrings.. here's an example:

http://localhost/api/identification?word=hey&&userId=12

this will work in your case so you won't need to change anything in the code.

And Finally my favorite solution: Create a model class like that:

public class IdentificationModel{
    public string word { get; set; }
    public string userId { get; set; }
}

then require it as a parameter object in you method like that:

[HttpPost]
public Probabilities Post(IdentificationModel requestModel) {
    Request request = new Request();
    request.Identify(requestModel.word, requestModel.userId);
    Probabilities probabilities = probabilitiesFactory.GetBy(request.ProbabilitiesId, "Id");
    return probabilities;
}

The last one does not require adding [FromBody] attribute since any class object is an api method will automatically be waited as a Request Body Object.