If you're already ordered, it's too late. They don't let you apply it retroactively.
It's $99/mo plus tax or a one-time fee of $8,000. Since you're leasing, it wouldn't make sense to buy it outright as the breakeven point would be beyond the scope of your lease. Of course, that assumes that the lease price remains the same. You can subscribe, and unsubscribe, one month at a time anytime you like.
It will take a few days to get used to FSD. At first, you won't be comfortable enough to trust it and will be disconnecting it when it was unnecessary. Don't give up. Once your comfort level increases, disengagements are rare.
Like I said, if something doesn't seem to be working right, just ask. Lots of YouTube videos out there to show you the ins and outs, too.