You want to apply, for a Sub-Task, the same Priority as its Parent Issue.
How it works
By defining this Workflow Function in a Transition of your workflow, you are able to force Sub-Task's Priority to be updated with Priority of Parent Issue.
Configuring the Feature
To configure the Inherit Priority Post-Function, please follow these steps :
Since this Workflow Function does not need to be associated with a screen containing the concerned CustomFields, the function can be added in any Workflow Transition. It might be:
A Common Transition,
Or a Global Transition.
Note that if you need to use such Post-Function in a Initial Transition (Create Transition), you have to check that the Post-Function is used :
After Create Issue Post-Function
and before index Issue Post-Function
If it is not respected you will see below message when trying to perform the transition.
Add the Inherit Priority Post-Function to any wanted transition.