In case, you are on a plan that support parent child ticketing, you will be able to enable it by heading to Admin > Support Operations > Advanced Ticketing and enabling the toggle for Parent-Child Ticketing. Once parent child ticketing is enabled in your account, you can trigger the form by clicking on the parent child tab from the right side panel.
If you wish to automate the creation of a child ticket based on certain ticket properties, it can be done by trigger a webhook. Here is a solution article with a detailed note: