The third-party authorisation process it is entirely controlled by eBay, it is their security check for your account and Optiseller quite rightly has no visibility of this. 

The two-factor authentication is something that is set-up via your eBay account settings and is not something that Optiseller has any access to.  The setting is in your eBay Account under Personal Information/Security Information, but this may vary depending on locale and seller-type.

There is also a known issue with a legacy 2FA on eBay accounts that may be related to why you are receiving the prompt for a six-digit security number.  Please refer to this TameBay post for more details.

Please review your eBay account and contact eBay Support if required.