Sorry, I didn’t see your post while I was composing the one I just posted or I would have answered there.
I’m not aware of any way to sign up ahead of time. You are welcome of course to investigate the church during this life and accept here if you wish. We are just as willingly and delighted to perform these saving ordinances with and for the living.
Presumably, if you have a family member who is LDS they will do the work for youat some point after your death. There is a waiting period, so it’s not like you can run right out after a funeral and perform the ordinance.