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.

Cisco Duo is a multi-factor authentication (MFA) solution that verifies the identity of all users with Duo's easy, one-tap-approval MFA app. It also provides device visibility and adaptive policies to ensure that only trusted devices can access corporate applications and networks. Some of the benefits of using Cisco Duo as an MFA solution are:
* It provides a simple and streamlined login experience for multiple applications and users. Users can access all their applications from a single dashboard, and authenticate with a simple push notification or other convenient methods. Duo also supports single sign-on (SSO) for integrated applications, reducing the need for multiple passwords and logins12.
* It offers native integration that helps secure applications across multiple cloud platforms or on-premises environments. Duo can be easily integrated with most major apps and custom applications, enabling a secure access solution that can be implemented with minimal IT involvement. Duo also supports various protocols and standards, such as SAML, RADIUS, LDAP, and WebAuthn, to enable seamless integration with different environments13.
References: 1: Multi-Factor Authentication (MFA) | Duo Security 2: Multi-Factor Authentication (MFA) | Duo Security - Cisco 3: What Is Duo? Two-Factor Authentication From Cisco - Cisco