How to restrict Github Post-Receive WebHook to master branch only

Question is pretty much self explanatory. I want that the webhook will be triggered on commits to master branch only, I might be wrong, but from my experience right now it seems to be triggered on commit to every branch.

How do you create a Webhook secret?

Adding a deploy secretSelect Configuration in the project settings panel. Scroll down to the Webhook Deploy Secret section, then click Set Webhook Secret. Looker will automatically generate a secret token. You can use this automatically generated secret, or you can type in your own secret token.

Considering that you don't have much control over a GitHub post-receive hook, it seems easier to filter the JSON message representing the payload of said hook.

You can check the value of the "ref" element