Enabling Github webhooks for multibranch jobs?
According to the official GitHub Branch Source Plugin documentation, the plugin can automatically configure webhooks for you if you have your GitHub API token configured in Jenkins global settings:


  You can change the build configuration to perform the folder
  computation at different times. The default setting will scan your
  GitHub repository for changes if it has not received any change
  notifications from GitHub within the past day. This scan will catch
  the repositories you’ve added or removed from the managed GitHub
  repository. You can always force the Folder Computation to run from
  the GitHub Organization page.
  
  While the build triggers are often enough, you can set up webhooks to
  automatically trigger builds when changes are pushed to your GitHub
  repositories. To do this you must have a GitHub login with a token.
  
  
  Go to the main configuration settings page, Manage Jenkins > Configure System
  In the GitHub Plugin Configuration section, add a server with your credentials
  If you need a token, generate one with the Additional Actions > Convert login to password and token
  
  
  You can also configure this manually through GitHub itself by
  registering the URL provided in the help section of the server config.