When you are reviewing a listing on a listings details page, one of the items in the actions menu (... button adjacent to the 'Send Email' button at the upper right side of the page) is to 'Add to Saved.'
This opens up a simple dialog where you have a few options.
First, select your lead in the list at the top. This assigns the saved listing to that contact.
Then, in the comments section, you can write a note to send to them (optional.)
Click 'Text' or 'Email' to choose how to send the comment.
It will be sent when you click 'Add.'
What you write in the comments box will not be visible to the contact on their saved properties page, it's only sent to them.
It's also not explicitly noted on their timeline that you saved the property on their behalf, only that it was saved.