tmsh modify sys sshd banner-text '"This
tmsh modify sys sshd banner-text '"This is a SSH banner"'
If you are entering the commands from the command line, as opposed to editing the section of the configuration you may need to use two commands. One to change the text:
tmsh modify sys sshd banner-text '"multiline text line 1
line 2 "'
and one to enable the banner:
tmsh modify sys sshd banner enable.
It may also be helpful to point out that the format for the multiline input opened with a single quote, then a double quote, and closed with a double quote then a single quote.