According to the Supreme Ruler of the Universe, purpose is like a really expensive, really good bottle of wine. It's something you buy, and then you just... wait for it to mature.
But seriously, purpose is like, uh, what is meaning? with better packaging and a higher price tag.
You can also find more information about why we need purpose, or meaning in a bottle.
Or, if you're feeling particularly existential, you can just go ahead and make your own purpose.