Hi Sarath,

Greetings from Freshdesk Community :)

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.

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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: 

https://support.freshdesk.com/en/support/solutions/articles/50000002311-create-a-new-child-ticket-automatically-using-a-webhook

Hope this helps!

Feel free to let us know if you would like more clarifications on the same.