Sending a message to the DAO can be done two different ways.
Via the DAO Website
Visit the page of the website.
Fill in your message information and click "Send Message". From there you'll need to sign the transaction.
Once the transaction is confirmed, the Smart Contract will emit an event with an IPFS hash of your message allowing DAO members to consume it.
Via Smart Contract
You may also interact with the smart contract directly by using the following ABI.
You'll first need to upload the file with your message to IPFS and retrieve the hash (_messageUri).