Under Settings > Account Information > Deactivate Account you're just sent to a customer service email form. They say it takes 1-3 business days to process.

As a side note, you have to close out all of your positions and transfer out all of your money before you're allowed to close the account. This seems unusual to me from experiences with other brokers.

From my understanding the average retail broker will allow you to transfer you assets out to another brokerage. You can do this as part of closing your account.

While just transferring money out separately and keeping the account open longer isn't a big deal, there's tax and other financial implications to having to close your positions instead of transfer them. You'll have to realize capital gains. You'll also have multiple banking days in which the price could move up while your money is transferred to another brokerage to re-buy the shares.