How to Send a Message using Siri on your Apple Watch
Step 1. After raising your wrist to wake your Apple Watch, say Hey Siri. Alternatively, press and hold the Digital Crown.
Step 2. Say Send a message to [contact/phone number]. You can also say something like “Tell [contact] headed home period”.
Step 3. If prompted, select the device to which you would like to send the message.
Step 4. Tap the microphone button.
Step 5. Speak your message.
Step 6. Press the Digital Crown to send your message or say “Hey Siri, send it.