By default, all comments posted on the contest page are visible to all users as well as contest holder. However, in the situation when you to send a message that you only want the contest holder to see, you can scroll down on their contest page to the bottom and select 'Send Message' on their About Contest Holder section. 

Please note: Using private messaging to gain advantage over other creatives is not allowed. If the response to your question or private message can also be helpful to other creatives, please ask this question in a  public comment. Asking such questions privately can lead to account suspension.