In this episode, Jessica Dale demonstrates how to send Messenger messages using Siri on iOS and iPadOS.
To set this up, go to Settings > Apps > Messenger > Apple Intelligence & Siri, and toggle the "Use with Siri requests" switch on. To send a message, engage Siri and say something like "Use Messenger to message [person]." Siri should then prompt you for the contents of the message and then offer to send it, as it does when sending a message using the built-in Messages app.