Originally Posted by flyertalk
There is the normal reply button, labeled "Post Reply", it is located at the top and bottom of each thread and requires no deleting of quotes...

Sounds like that should work for you.
--Tim
This is correct when replying to a normal FlyerTalk thread, Tim, but
500 miles at a time specifically inquired about replying to a
private message, where there currently is no “Post Reply” button.