Android- login RESTFUL api which returns an id when login succeeds

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Java

Android

REST

API

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I have made a login api which will return an id when i pass username and password in android app.

public class LoginApiBody
{
    public string UserName { get; set; }
    public string Password { get; set; }
}

Say the link is http://abcd.org/efg/login. Currently the link is showing:

{  
   "message": "The request is invalid.", 
   "messageDetail": "The parameters dictionary contains a null entry for parameter 'Id' of non-nullable type 'System.Int32' for method System.Net.Http.HttpResponseMessage Get(Int32)' in 'KnwPortal.Controllers.UsersAPIController'. An optional parameter must be a reference type,  a nullable type, or be declared as an optional parameter." 
}

I have to pass 'UserName' and 'Password' to the server with POST method.My question is in which format i should send/pass my username and password? whether in JSONObject or something else? how he can verify whether the passed UserName and Password is there on the server or not?
or i am doing something wrong with my loginapi?

What I have tried:

I have tried this, but no success

Ion.with(User_Activity.this)
        .load("http://abcd.org/efg/login")
        .setBodyParameter("UsserName",user)
        .setBodyParameter("Password",pass)
        .asString()
        .setCallback(new FutureCallback() {
            @Override
            public void onCompleted(Exception e, Object result) {
                //on complete
            }
        });