In this widget you have to type the Property ID you’d like to show and a second line. The Property ID is taken from the URL of the Property Page in Admin – Properties – Edit Property.
EX: In this URL http://http://wprentals.net//wp-admin/post.php?post=90&action=edit the property ID is 90.
There is another way to see IDs easier. Please see this help – http://help.wprentals.org/2015/10/28/how-to-see-ids/