Default cost basis?

Let’s say you have some bitcoin in cold storage and you send some to your exchange account in order to sell it for fiat. By doing so I believe you create a taxable event. If so what does the exchange use as your cost basis for determining a capital loss/gain?

Or do I have to manually do it myself and use the FIFO principle when filing taxes?

Edit: I’m not actually selling any bitcoin but I’m curious how that would work from a tax/cost basis perspective.