The bardic focus:
When you use the instrument to cast a spell that causes targets to become charmed on a failed save, the targets have disadvantage on the saving throw. This effect applies whether you are using the instrument as the source of the spell or as a spellcasting focus.
And the spell:
Duration: Concentration, up to 1 minute
Creatures of your choice that you can see within range and that can hear you must make a Wisdom saving throw. A target automatically succeeds on this saving throw if it can't be charmed. On a failed save, a target is affected by this spell. Until the spell ends, you can use a bonus action on each of your turns to designate a direction that is horizontal to you. Each affected target must use as much of its movement as possible to move in that direction on its next turn. It can take its action before it moves. After moving in this way, it can make another Wisdom saving to try to end the effect. A target isn't compelled to move into an obviously deadly hazard, such as a fire or pit, but it will provoke opportunity attacks to move in the designated direction.
The spell doesn't outright say the creature is charmed, but also states the spell doesn't work on creatures that can't be charmed, and the effect of the spell is similar to that of a charm spell (and uses the same saving throw to prevent it).