Preconditions allow you to execute a post function only under certain circumstances.Add the Value Field precondition to check if a field in the origin issue matches a specified value.

How to set up the Create a Linked Issue post function

Learn how to set up a rule to:
1. Create a linked issue.
2. Copy the last comment from the origin issue to the linked issue description.
3. Use a precondition to run the rule only when the origin issue has an assignee.

Post function configuration: Create a Linked Issue (JSU)