How to specify endpoint for server-side validation of Facebook Audience Network's rewarded ads?

Question

I'm trying to integrate Facebook's Rewarded Video Ads into my app; specifically, I want to implement server-side validation so that I can verify that the user has watched an ad video before giving them a reward. According to the implementation details as specified on this page, this involves having a custom endpoint on our end which Facebook will call with appropriate query parameters each time an ad is watched. The documentation says that I need to "provide your publisher end point to your Facebook representative in order to enable this feature." Any ideas on what this means or how I can specify the endpoint?