Custom Restangular GET method

Question

I'm making a GET request to my web api, the url goes like /users/exist/:username

I've created the custom method that works:

RestangularProvider.addElementTransformer('users', true, function(user) {

    user.addRestangularMethod('exist', 'get', 'exist');
    return user;
});

But I don't know how to add the :username in the url.

Is it even possible?