You can insert custom fields to your email templates or email notifications by clicking on Personalize on the Visual Editor screen when you edit templates.
Click on Edit by one of the notifications. In this example we will click on edit for "New Ticket - Public Users." Click on Personalize Message on the next screen.
When you click on Personalize Message you can choose to insert any of the Standard Personalization tags show before or you can insert any custom personalization tags that you create for Ticket Fields, User Fields, and Admin Fields
Furthermore, you can add custom personalization tags to the email templates of any Automations that you create.
