There is a slack message module in the app store, maybe you can use that?
Well, that usually takes only 60 minutes. At least according to this live build session :-)
Hope this works for you.
Btw. It is native, not yet PWA
You could also look into the pusher module, one of the typical use cases for this module is a chat function (https://docs.mendix.com/appstore/modules/pusher)
Please find the link below for the video of configuring the chat system:
Please mark answer accepted if you find it useful.