You can check the setup key when you set up the Two-Factor Authentication (2FA). Please check the 2FA set up procedure here: https://support.bitcastle.io/hc/en-us/articles/14181836264729-How-do-I-set-up-Two-Factor-Authentication-2FA
Also, please note that it will not be displayed again.
If you have further inquiries, please don't hesitate to contact our Support Team via "Contact Us" function.