How to send a message via Apple Watch

Read the following

https://support.apple.com/guide/watch/send-messages-apd81334ff71/watchos

If you are talking about traditional SMS/MMS TEXT messages, then read the footnotes on this webpage

https://support.apple.com/en-us/HT205547

1. To receive SMS, MMS, or push notifications from third-party apps on your cellular Apple Watch, your paired iPhone must be powered on and connected to Wi-Fi or cellular, but it doesn't need to be nearby. You also need to be signed in to iMessage on your iPhone.

According to Send messages from Apple Watch - Apple Support:

Create a message on Apple Watch

1. Open the Messages app  on your Apple Watch.
2. Scroll to the top of the screen, then tap New Message.
3. Tap Add Contact, tap a contact in the list of recent conversations that appears, or choose an option:

• • Tap  to search for someone in your contacts or to dictate a phone number.
  • Tap  to choose from your full list of contacts.
  • Tap  to enter a phone number.

1. Tap Create Message.
2. If you've set up your Apple Watch to use more than one language, tap Language, then choose a language.