Requirement:

At the moment, when the issue is linked between Parent-1 and PartTask-2 using link type 'is action by', linked by becomes Parent-1 actions PartTask-2. When they are linked, automation with copy forms is trigged using PartTask-2, so it becomes triggerissue or main issue and Parent-1 becomes the destination issue.

If we need to copy the form from Parent-1 to PartTask-2, it isn't possible because the when the rule is triggered with Copy Form action, we get Trigger Work Item or Current Work Item, and branching doesn't help because we don't have copy from 'Destination Work Item'