Is the action to maintain a Witch Bolt the Cast a Spell action?

@JeremyECrawford is the action to maintain a Witch Bolt the Cast a Spell action? Could I cast misty step on the same turn I maintain it?

— (((Em Wizardshark))) (@DylanRubini) June 28, 2016

You use the Cast a Spell action to cast a spell. An action in a spell that's already cast is an ad hoc action. #DnD https://t.co/uIURQEw0NC

— Jeremy Crawford (@JeremyECrawford) July 21, 2016