I know that I can have a subtask inherit a value from its parent by using the Copy Field postfunction and choosing the same field name for both source and destination, then choosing Copy from Parent.
This works fine when the field being copied isn't a required field. But the Account field is, for us, required at the time of issue creation, and when I tried to use this postfunction, the create screen for the subtask doesn't automatically select the Account value from the parent.
Parent:

Subtask creation:

How can I accomplish this?