Click 'Add new address' in the 'Destination Address' field
A 'New withdrawal address' window will now pop up:
Enter your external crypto wallet address and choose Comment (you can enter anything you wish in the 'comment' field to help you keep track of your withdrawal addresses)
Click 'Add'.
In the next pop-up window, enter the last PIN code that you receive in your email inbox.
Once you have confirmed a withdrawal address it can then be used for withdrawals