How do I add a legacy contact for Apple ID ?

I don't think you have to do anything formally. You should, however, receive a message or at least a printed copy of the access code for when you need it.

See How to add a Legacy Contact for your Apple ID - Apple Support

There's also more detailed information at https://www.macrumors.com/guide/legacy-contact/

Limnos, Thank you for the link you provided to the Legacy Contact information. I was given the permission from my wife and unfortunately I now have to utilize the link. You have been helpful in this difficult time. Bless you 🙏

You're welcome. My commiserations.