Add Assignee to subtaks


I’ve tried adding a variable to assign my parent task Assignee to certain subtasks when created, see below:

This rule only seems to be working if an Assignee is already assigned and the rule is triggered.

Our workflow means submission come in via a form, with the trigger selected, then we assign ourselves. I would like the variable to trigger when someone is assigned, not only if someone is already assigned.

Any tips or work arounds?