I have the exact same issue - sometimes I want to suggest another of my listings, or maybe I want to refer them to a local host. I find that trying to explain to them how to copy and paste the listing ID on to the back of the url, etc etc to be too complicated for a large part of the demographic that inquire about my listings. I try various combinations - with the https, without, just the listing ID, the whole link - it's pretty ridiculous that there isn't a filter to recognize a legitimate existing url. I finally then have to walk people through going to my full profile page, looking for my listings and navigating that way. Works if it's my own listings, doesn't work if you're referring the guest to someone else.