When I create a sub-task, can it be possible the inherited custom field already set inside the textbox instead of empty field?
You can achieve this using several of the plugins. See https://scriptrunner.adaptavist.com/latest/jira/builtin-scripts.html#_clones_an_issue_and_links for an example of how this is done with a clone of an issue. You can apply the same principles to creating a subtask.
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