That is correct: you can not send your promo code in your rover messages. Also promo codes can not be combined with other offers they might have from signing in to rover.com or if they have use rover before with another sitter and used a code before it will not allow them to use your code.
Instruct them to enter the code when confirming the stay (paying) by clicking on their inbox and then on the request they sent you; on the left side of the messages.
The easiest way to give out your promo code is during the meet ups by giving them your business card; if you haven't order any- you might want to do so by using the sitter store and following the instructions, they pay for themselves as you also get money ($10) if they use your promo code when booking with another sitter. (When I was new to rover I waited until I had completed a few stays before I invested in it)
Sometimes customers prefer to talk on the phone and you can tell them then, just by telling them that you can give it to them during the meet up or if they seem interested in paying/not doing a meet up you can have them write it.
Most customers don't even show interest in the promo code if you give them a good deal.