Here is the link to our article on Adding/Modifying service form fields for Projects (https://solutions.teamdynamix.com/TDClient/KB/ArticleDet?ID=4696
This may help you get your service form modified as needed.
If it is a Ticket form, you would instead go to Admin > Ticketing Applications > [your ticket app for the service] > Request Forms > [locate request form used for service]. If the service did not use a form created from Admin, and was instead manually created, you would go to your Service Catalog, locate the service and choose to edit it. Go to the Form tab to modify the form. Then find your field on the form, choose to make it "Required" and save the form.
I hope this has clearly provided instruction on how a form in a Service Catalog service can be modified.
Please let me know if you need any other assistance with this.