I've been having trouble with this also.
The only thing that works for me is to dictate the text after saying "Hey Siri, text [person] [the message I want to send]" and then saying "hey siri, send text" once the send/cancel option screen shows.
However, even this is temperamental because it sometimes sends the text when I say that, and sometimes it doesn't. Worse, sometimes it will not show that it has sent the message and leaves the send/cancel screen up despite having sent it, so I say "send text" again, and the person receives the text twice.
I'm hoping this gets tightened up with future updates, but thanks to this thread, I have been able to make it work at least some of the time! Thanks, guys.