Below script can be used to create a number sequence under Request Additional Field using Request Custom Function and Business Rule.
* Add this newly created additional field to all the request templates available.
* Navigate to Admin > Developer Space > Request Custom Function > Copy paste the attached script and Modify the highlighted entries.
* Additional field API name will be under Admin -> Incident additional field API Field Name.
* Update the Number Sequence besides CustomReqID
Configure the business rule as in the below: