public class CredentialsAuthProvider : ServiceStack.Auth.CredentialsAuthProvider
{
    public override bool TryAuthenticate(IServiceBase authService,
        string userName, string password)
    {
        //Add here your custom auth logic (database calls etc)
        //Return true if credentials are valid, otherwise false
        if (string.Compare(password, "123Eps123", true) == 0 && string.Compare(userName, "JBEps", true) == 0)
            return true;
        else
            return false;
    }

    public override IHttpResult OnAuthenticated(IServiceBase authService,
        IAuthSession session, IAuthTokens tokens,
        Dictionary<string, string> authInfo)
    {
        //Fill IAuthSession with data you want to retrieve in the app eg:
        session.FirstName = "administrator";
        //...

        //Call base method to Save Session and fire Auth/Session callbacks:
        return base.OnAuthenticated(authService, session, tokens, authInfo);

        //Alternatively avoid built-in behavior and explicitly save session with
        //authService.SaveSession(session, SessionExpiry);
        //return null;
    }
}

Plugins.Add(new AuthFeature(() => new AuthUserSession(),
            new IAuthProvider[] {
                new KCRouterLibreary.CredentialsAuthProvider(), //HTML Form post of User/Pass
            }
        ));

var _authenticate = function (sUsername, sPassword) {
        var parms = {};
        parms.userName = sUsername;
        parms.password = sPassword;
        parms.rememberMe = true;
        var promise = $http.post("http://" + sIP + ":20001/auth/credentials?format=json", parms).then(function (response) {
            var result = response.data.result;
            this.sessionID = response.data.sessionId;
            $http.defaults.headers.common['Authorization'] = 'Basic ' + this.sessionID;

            initEventSource();

            console.log("Logged in: " + this.sessionID);
            return result;
        });

        return promise;
    }