Activate Witch Bolt is classified as a cantrip and will benefit from cantrip exclusive boosts like the Necklace of Elemental Augmentation. On the other hand, it will not activate abilities like Tempestuous Magic that require casting leveled spells.
Unlike the initial cast of Witch Bolt, Activate Witch Bolt has no Attack roll. The damage is guaranteed.
If the spellcaster is maintaining Witch Bolt on multiple targets (through Metamagic: Twinned Spell), Activate Witch Bolt will damage each target.