There's no defined on-site or on-form action an Engagement campaign is optimizing for, the entire point is interaction with the post itself, not what happens afterward on a website.
Ask your agency this instead
Is this engagement campaign meant to warm up an audience for a future Sales or Leads campaign? If so, how will we know it worked?
Want the metrics that actually matter for a Engagement campaign? Open the tool and pick one.
Adamigo provided the Engagement-objective CTR, CPC, and CPM figures used here, along with explicit engagement-rate tier language that informed how the bands were set.