Hasura : Using environment variable in event trigger

Hasura says Note: Specifying the webhook URL via an environmental variable is recommended if you have different URLs for multiple environments.. But couldn't find a way to put environment variable in the webhook url.

Then I found this url https://hasura.io/blog/moving-from-local-development-staging-production-with-hasura/ , turns out we can use environment variable this way. {{FIREBASE_FUNCTIONS_API}}/users/