A living will is a document, typically signed in advance while in good health, that specifies the decisions somebody wishes to be taken about his or her medical treatment in the event of becoming incapable of making such decisions or communicating choices.
The short answer is: no, you are not obligated
to have a living will. However some people choose to have one to be certain that their wishes in certain medical situations are followed.
The National Catholic Bioethics Center has some information on how Catholic medical ethics intersects with living wills: A Catholic Guide to End-of-Life Decisions