To setup a TOTP or Two Factor code, create a new entry or begin editing an existing entry and select Add TOTP. You can then either scan a QR code or enter the secret or OTPAuth URL manually.
Strongbox uses KeePass custom fields (compatible with KeepassXC (“TOTP Seed” and “TOTP Settings” or the KeeOTP (“otp”) plugin). If you already have these configured, Strongbox should just work.
If you enter an OTPAuth URL as the Password field, or anywhere in the Notes field (handy for Password Safe users) then Strongbox will automatically pick this up and display the TOTP code.