When you create a Custom Form for Incidents, that form will not appear on Service Requests. The Custom Form seems to only work when a user is filling out a New Ticket and not a Service Request.
I have created a new Custom Form for Incidents. I have created a dynamic form rule to show the form without any conditional logic. When going to Service Request, the fields on that Custom Form do not appear for the user to fill out.
I've set all fields to "Show in Portal". It works if I go to New Ticket, but when I attempt to create a new service request, the form is not shown. However, once the request is in, the fields will appear on the backend for the admins to complete.
So it seems if you want a person to fill out any information, you will need to configure all fields on the Service Catalog item? There's no way to reuse Custom Forms on Service Requests where the user creating the request can complete the Custom Form.