Follow the below steps to get a solution as per the requirement
1. Update the default priority calculation matrix i.e.
2. Login to Service Desk Manager - Administration Tab - Service Desk - Request/Incidents/Problems - Priority Calculation
3. Select the 'Default' record and Edit
4. Under Priority Calculation Options - update Urgency Default value to '4-Soon'
5. Save and Test the behavior from Catalog by raising on 'Report and Issue'
1. If Urgency checkbox is checked, the priority is set to 4
2. If Urgency checkbox is unchecked, the priority is set to 3