When player is offline

When a player is offline, how do I make it so they get a specific currency?

What do you mean by this? Kinda vague.

You could use some events and increment some currency on those events.
https://developer.roblox.com/en-us/api-reference/event/Player/Idled
https://developer.roblox.com/en-us/api-reference/event/UserInputService/WindowFocusReleased

You could save last time they joined the game and calculate the amount of offline currency they get from joining again.