Groovy Automation Project

Problem statement:
I want to set the custom script post-function and behaviour to increment the custom field’s (text field) by +1 on a specific workflow transition.
Program (e.g..10145667)
Lable (e.g.. 45667)
External (e.g.. 1)
Means, when the specific transition is fired from the status fixed –
A. Create a new Jira Issue with the same Issue Type within the same project
B. Set the Program field to 10245667 (e.g.. +1); Lable field will remain same, External field will be incremented by +1
C. Validate the Program field value in Existing jira issues (e.g.. using JQL) if it does exist than populate the exception “Program number exist in Jira issue”

Version: Jira 8.20.6