a Cisco FirePower sensor to Firepower Management Center is configure manager add <host><key>. This command establishes a secure connection between the sensor and the FMC using a registration key. The <host> parameter can be either the IP address or the hostname of the FMC. The <key> parameter can be any alphanumeric string that matches the one configured on the FMC. This command is executed on the sensor's CLI in the privileged EXEC mode12. References: 1: Implementing and Operating Cisco Security Core Technologies (SCOR) course, Module 2: Network Security, Lesson 2: Deploying Cisco Firepower Next- Generation Firewall, Topic: Registering the Cisco Firepower NGFW to the FMC2: Firepower Management Center Configuration Guide, Version 6.6 - Device Management Basics [Cisco Secure Firewall Management Center] - Cisco.