@Murat and turkandjayjee do you mean you want to input a withdrawal address? Why would you want to input a deposit address when it should be the site giving it to you?
You will need to provide your withdrawal address when you generate your deposit address for the first time. I think this helps even if your account gets hacked, other parties expect you won't be able to withdraw your funds to their address. Though you can change your withdrawal address by contacting the support team. You may need to verify ownership or provide the necessary details to change your address.