Hello!
We had the same query, took us 15 Rounds of messaging ABNB CS, to learn:
48 hours prior to arrival….Guests are able to view checkin instructions we have completed on our listing, it is not messaged to them…it is simply available for view.
We have auto messaging set up for check in instructions, and many other things as well, 95% success rate, we always verify for the critical information, such as check in/check out instructions.
Keep in mind, in our experience 70% of the time guests do not read anything sent and ask instead…..