hello @lioness,
You have the right goal in mind- sharing your wishes with loved ones who can help make sure those wishes are known in the event you are unable to express them yourself. When people have shared that they are having trouble bringing up the topic or attempted to, but loved ones weren’t ready, I encouraged them to find an opportunity to elaborate on a situation seen in a television program, movie, or current event. You would think that it would or should be somber but it doesn't have to; it just needs to be a way for that individual to relate to & understand the importance of starting the talk now. It may be taking the focus off you and asking an adult child if they ever thought about the care they would want or not want in the event they couldn't express it for themselves.

I used an episode from the old “ER” program to help my husband & adult daughter understand better. It is a serious portrayal of a woman who suffers a stroke-from the patient’s perspective. Youtube has a clip that contains two brief back-to-back clips (watch them both). It is dated & a little fuzzy but the message is crystal clear. It is called- “ER” Alone in a Crowd (Cynthia Nixon).
https://youtu.be/QnYgO01r9LE

If you're seeking a lighter, more comedic way to initiate the conversation, I would be happy to share the name of a clip from the sitcom Seinfeld.
I hope some of this helps; don’t give up- keep plugging away!