Unfortunately, as of January 2017, this workaround has been closed by FB. The checkbox mentioned above is no longer there, and despite exhaustive searching, there appears to no longer be ANY way to defeat or disable the "press enter to send" feature. So FB users will continue to be plagued and confused by half-completed PM messages that there is no way to recall. The only alternative appears to be that of asking your correspondents for a "regular" email address, and abandoning FB Messenger for the purpose of private messaging. I used to think that if enough people complained about FaceBook's non-intuitive, user-unfriendly features, the company would listen, and fix them, but millions of people have been complaining, for many years, about Press Enter To Send, and FB has not only steadfastly refused to fix this problem, they outright insult users who complain by sending bland, non-helpful replies that side-step the issue. These usually suggest that we just need to learn to press "shift-enter" to get a line feed. Yes, that works, but it doesn't address the issue, which is that anytime you press enter, the message gets sent, and all too often, it's an incomplete message!