. . . wait a minute . . .
that’s the button that you press to send the message that you’re composing?
Now I’m not sure what button I thought that was at all.
I think “send message” is too long, butI think “Send” would make sense. 
(Below, I finally figure out that “Send message” isn’t much different from “create topic”)
And there is a ton of space, so “Send Message” would certainly not crowd anything, at least on Desktop.
The “reply” button in the composer makes perfect sense as “reply” . . .
The “create topic” button in the composer says “create topic”, which isn’t different from “Send Message”, so yeah, I was an irrational curmudgeon.
I stand by my comments about “website” and “backslash”, but am reversing my previous stand and saying that “Send” or “Send message” would be better.