Use a ScriptRunner event listener to listen to all issueUpdated events in JIRA
- Use an event listener to listen out for the custom field value to change
- If the custom field is updated, then set the event listener to create a sub-task
- Choose JIRA Admin --> Script Listeners --> Create a subtask --> Expand Examples
- Use the "check for field changes"
The temptation in this instance may be to set up a post function during the create transition (see the related Atlassian Answers) but that won't help for changes after the issue is actually created.