I found there is the data-bind-value
method.
<form data-formfor-message="controllers.messages.newMessage" data-event-submit="controllers.messages.create">
<input placeholder="type here..." data-bind="message.body" />
<input data-bind-value="message_thread.id" type="hidden" data-bind="message.message_thread_id" />
<input data-bind-valude="currentUser.id" type="hidden" data-bind="message.user_id" />
<input type="submit" value="Reply" />
</form>
That does the trick.