With a REST Hook your API needs to implement a subscribe endpoint that receives and persists a callback hook URL (unique to that Zap runtime instance). Use that URL for hook callbacks. You’ll also need to implement an unsubscribe endpoint that removes that subscription.  

This is important as a user may use your trigger in several different Zaps to effect different workflows.  This also means your system needs to support multiple hook subscriptions for the same event. 

For users building Zapier integrations that they don’t intend to publish, there are static hooks, where the user has to cut and paste the callback URL from the Zap Editor into the app’s webhook configuration UI. However this is still going to be different URL for every Zap a user builds. To build a static hook, leave the subscribe and unsubscribe endpoint undefined.

Again, if you are planning on requesting that your integration be published in the Zapier App Directory, don’t use static hooks. The app review team will have to reject your submission. We’ve found they result in poor Zap activation rates and lots of tickets to support. REST Hooks are the way to go - better for everyone involved.