You are working with Ticket Form Template but noticed that the Product External ID Field is not visible.
SAP Cloud for Customer
This field is currently no enabled for Form Template and cannot be added to Custom Forms as well.
You can add the idea to have the field enabled on the SAP Customer Influence Site.
Form Template ; Summary ; Preview ; Generate Summary ; Form Preview ; Template ; Form Template Maintenance , KBA , LOD-CRM-SRP , Service Request Processing , How To
SAP Cloud for Customer add-ins all versions