It works similar to what we use in the app. All you have to do is to ask someone from your repo admins with enough permissions to click on the link below. It will ask to allow access to the repo for our GPush GitHub app through OAuth authorization, which is pretty convenient and super safe. In the result of it GitHub will issue access token which will be used only once for webhook creation. After you've seen the success message - make sure the webhook has been created.
Navigate to https://github.com/OWNER/REPO/settings/hooks. Click on the Add webhook button. On the settings page that opens you should use the following settings. After you're done with copypasting - click on the Add Webhook button and you're all set!
Feel free to use any http-client of your choice. If you don't know what to use - these are good examples: CURL, Postman. In order to make a successful request you need to use special headers and send a JSON request.