There are two ways you can message a creator you are supporting on Patreon:
From the "overview" section of a creator's page, click "more" on the left-hand side and then "message."
Navigate to the drop-down menu on the top-right while signed into Patreon.com and click "messages."
From there, you can enter the name of any creator you are a patron to into the text field and you'll see a message box pop up.
Note: If you have been blocked by a creator you will not have any permission to message the creator.