You can use the create subtask transition post function to create subtasks when the issues are first created. 

Steps

1. In the Administration > Issues > Workflows page, select a workflow and click Edit.

2. Go to the Open step.

3. Click the Create Issue incoming transition.

4. In the Transition: Create Issue page, go to Post Functions.

5. In the Post Functions tab, Click Add post function.

6. In the Add Post Function To Transition table, select the Create Sub-task (Create on Transition) option.

7. Click Add.
   The Create on Transition for Jira - Create Sub-task(s) page is displayed.

8. In the Conditions & Configuration page, select the Create if original issue is a sub-task option to prevent looping.

9. Set the other parameters as required.

10. Click Add.
    The Create Sub-task post function is added to the workflow.

11. Move the Create Sub-task post function below the Creates the issue originally. post function.
    When you create a parent issue using this workflow a subtask will be created.

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