Need Category Selected in HR helpdesk automatically assigns to queue

Summary:

When an employee submits an HR Help Desk Request without selecting a Category, the Category field remains blank on the created request.
We would like to default the Category to “Uncategorised” whenever no category is selected during request creation.
Could you please advise on the recommended configuration to achieve this in Redwood HR Help Desk? Specifically, can this be handled through a defaulting rule, Groovy trigger, or any other supported configuration so that requests created with a blank Category are automatically populated with Uncategorised?
I have already configured the assignment rule using the employee’s Business Unit as the routing attribute. However, although the request is being evaluated for queue assignment, the Category field still remains blank.