Summary

This article would help you with the idea of marking subtasks as Linked issues on the parent issue with "relates to" relation. Please note that the suggestion made here is on the best-effort basis please consult Partner for optimal and reliable solutions.

Environment

Jira cloud 

Solution: 

Option 1: For the existing issue you can use a Scheduled trigger

Trigger: Scheduled (JQL : IssueType != Subtask)

Branch: Subtask

Edit Action:  Link issue - "relates to" to Trigger Issue

Here is the snippet of the configuration for reference:

Option 2: Trigger automation rule when subtasks created

Trigger: when issue created

If condition: Issue Type equals Sub-task

Add action : Link issue to : Relates to : {{issue.parent.key}} 

Here is snippet for reference: