Updated: Oct 18, 2022
I know the secret of immortality today. If you want to be Immortal, put your heart and soul into something, it may be for the service of parents, Guru, Nation, friends, society, or the whole of mankind, or put your heart and soul into any endeavor which is worthwhile for you, with intentions of love and compassion. There is no physical elixir of immortality but you will get the elixir by doing your great work the “magnum opus”, The elixir of mortality is nothing but divine knowledge which breaks the bounds of mortality, which you will get when you act with intention prescribed above.
The transition between mortality and immortality is a subtle one and you will know only when you have transcended to a different dimension. The whole of Universe shall be at your service.
Bhishma got the Boon of death by choice because of his unmatched service to his father.
Hanuman got the boon of immortality because of his selfless service to Rama and there are many examples in different cultures. I will not count the examples but only reveal the truth for you.
Bhagwat Geeta also says the same thing about selfless service. It's just that your karma need to break the bounds of mortality to become immortal and once you become immortal all the powers of the universal will gradually come to you and you will live in heavens, while enjoying the virtues of earthly dimension and you can visit heavens at your own will.