It is on Tip of the Spear, after Rally Point Alpha. After clearing the bridge and going up and around, you will hear that you have a side mission which is to kill a red Zealot Elite. Once you reach an Elite on the turret, the Zealot will be on the other side, near the back. Kill him with whatever you got, wait a couple seconds and BLEEP BLOOP!
Video courtesy of RoosterTeeth