When an issue is created without an assignee selected, the issue will be assigned to the project lead of the project.
Select Current issue.
The current issue will be updated.
Select Assignee.
In the popup field:
Set the Value * to Set field value manually (parser expression) and Insert the following expression:
%{issue.project.lead}
The project lead will be set as an assignee of the issue.
Choose which user* will be able to execute the post function. By default, it is set to the Current user.
Insert the following expression:
%{issue.assignee} = null