Sub-task is not a field. Sub-tasks are issues in their own right and there's a whole panel dedicated to them on their parent issues.
I'm not sure quite what you're looking for, unless you just mean "put a link to "create subtask" on the main issue display"?
I’m a designer on the Jira team. For a long time, I’ve fielded questions from other designers about how they should be using Jira Software with their design team. I’ve also heard feedback from other ...
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