This quest resembles The Institute's Mass Fusion quest, now seen from the perspective of The Brotherhood.
After the conclusion of Tactical Thinking, speak with Proctor Ingram. This is a key fork in the story, where you can either proceed with the quest by boarding the Vertibird or you can warn The Institute about this mission. Choosing the latter will make you a permanent enemy of The Brotherhood. Also note that this mission will expose you to a ton of radiation, so either suit up with power armor or carry a ton of Radaway with you.
As you travel to the roof of the Mass Fusion building, clear out the Synths with the Vertibird's gun. Then jump onto the rooftop and get ready for a lot more Synths.
Follow the waypoint indicator to the Executive Research Lab terminal, and access its files. You'll learn that your next destination is in the basement. Take the password contained in the nearby holotape and the executive ID on the desk in the adjacent room. Then take the cylindrical elevator down.
Follow the waypoints to the Mass Fusion Labs key and then into the Lab. After decontamination, head into the reactor chamber, unlock the agitator receptacle, and take the beryllium agitator. As you escape, prepare to take on a ton of automated security. Then take the elevator up and leave the building. The last step is to travel to Boston Airport and report to Proctor Ingram.