You can just go to the site and find the person and the copy the url over to the links area on the person's profile and you can add it to the citation page. If ancestry or findagrave get it into the search than you can attach it at a later date.
I like to attach the items for my family. The hint system responds to the people who save an item to a tree. If you are the first to save an item you set the hints for other people. This can be a good thing or a bad thing. Getting there first and more importantly attaching it to the -correct- person helps some of the newbies who follow the hints. Hints will still be made when other people attach it later but the first person does have the first chance to set the direction.