As some other users have already noticed, there is no way to select your number porting on the website at the time of ordering a new iPhone 14 Pro to become qualified to receive AirPods and 200 GC.
Some suggest here https://community.visible.com/t5/Need-help/How-to-transfer-phone-number-to-visible-while-buying-the-... that you need to select a new number at the time of ordering and when you have the physical device delivered contact CS to initiate the port. Others mention the option of porting over the newly assigned number exists in the app. There are no details in T&C of the promo that you will still be eligible if you select a new number, activate it and only then port your old number.
If anyone successfully was able to do this, e.g. buy a new device, activate the line with the newly issued number, and then get the old number ported while still remaining qualified for the promo, please comment here with your experience.