Create sub-tasks and link them to the parent or current issue that has a specific priority of your choice.
Choose Single issue.
Choose selected issue type and sub-task as its value.
Choose current issue.
"Type in an appropriate summary for the issue you are creating."
This will show up in the popup window. Choose current issue.
Insert the following expression:
%{issue.priority} = "Insert your desired priority here"
For more information about this feature, visit the following page.