View Adapter for vROps requires two keys, both of which are included with the Horizon Enterprise license. The Horizon Enterprise license that you put in the connection server license the adapter, and an additional set of licenses licenses the manager. It's basically done this way so folks can deploy vROps for their Horizon environment, but don't own vROps for their server environment.
This article is the best description available, but it somewhat tricky to understand.
VMware KB: Upgrading licenses to VMware vRealize Operations for VMware Horizon 6.0
