Explanation: To let FMC manages FTD, first we need to add manager from the FTD and assign a register key of your choice. The command configure manager add 1.1.1.2 the_registration_key_you_want, where 1.1.1.2 is the IP address of the FMC, you need to use the same registration key in FMC when adding this FTD as a managed device. Reference: https://cyruslab.net/2019/09/03/ciscocisco-firepower-lab-setup/