By default the "Requested by" field will not be filled only the end user, if you have Service Desk integrated only with Unified Self Service, configure the data source of Service Desk, so it displays the "Requested by" field, then when creating a request, you will need to choose the contact desired.
1. Log in an USS administrator (CASMADMIN)
2. Go to Settings-> Data sources-> Ca Service Desk Manager
3. Click on Configure Now
4. In the bottom select the field " Requested_by" and click on Add a field
5. Configure the field so it is displayed and also required if it is the case.
6. Save the changes, next time that you try to create a new request the field will be prompted.