I’ve found the easiest way from a mobile to get the web version of Messenger, is to visit the normal Facebook website and request the desktop version. From here you can access the web version of Messenget by following the normal messages option.
A direct link is: https://www.facebook.com/messages/t/TextSpaced make sure to request the desktop version after following this link.
From a desktop just visiting www.messenger.com will bring up the desktop version. I hope that helps.