https://www.cisco.com/c/en/us/td/docs/security/ise/1-4-1/admin_guide/b_ise_admin_guide_141/b_ise_admin_guide_141_chapter_01110.html

Explanation : 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/ 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.
Explanation : 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/

Configure a Crypto ISAKMP Key
In order to configure a preshared authentication key, enter the crypto isakmp key command in global configuration mode:
crypto isakmp key cisco123 address 172.16.1.1
https://community.cisco.com/t5/vpn/isakmp-with-0-0-0-0-dmvpn/td-p/4312380 It is a bad practice but it is valid. 172.16.0.0/16 the full range will be accepted as possible PEER
https://www.examtopics.com/discussions/cisco/view/46191-exam-350-701-topic-1-question-71-discussion/#:~:text=Command%20reference%20is%20not%20decisive,172.16.1.128%20cisco123%0ACSR%2D1(config)%23 Testing without a netmask shows that command interpretation has a preference for /16 and /24. CSR-1(config)#crypto isakmp key cisco123 address 172.16.0.0 CSR-1(config)#do show crypto isakmp key | i cisco default 172.16.0.0 [255.255.0.0] cisco123 CSR-1(config)#no crypto isakmp key cisco123 address 172.16.0.0 CSR-1(config)#crypto isakmp key cisco123 address 172.16.1.0 CSR-1(config)#do show crypto isakmp key | i cisco default 172.16.1.0 [255.255.255.0] cisco123 CSR-1(config)#no crypto isakmp key cisco123 address 172.16.1.0 CSR-1(config)#crypto isakmp key cisco123 address 172.16.1.128 CSR-1(config)#do show crypto isakmp key | i cisco default 172.16.1.128 cisco123 CSR-1(config)#