Where is the best place to insert findagrave link? Can I enter just the ID #‽
Many people attached the Find a Grave record that is in the collection within FamilySearch to the person's sources: https://www.familysearch.org/search/collection/2221801
Or, they create a custom source with the Find a Grave URL to the memorial.
Some people create a Custom Fact on the person's detail page in the Family Tree.
Some people list it in the reason statement for the burial place. And some people also create a note and put it in a note.