how can I copy the value of the Organization field into a custom field during the Create of a new Issue?
do you mean from an add-on that you are developing? Or using any existing add-on?
standard development within JIRA.
You might need to ask this question over at the Usrr Community - https://Community.atlassian.com since they’ll know more about the day to day tricks.
We can help with how to write an app to do it but I’m not sure if that’s what you’re after.
I’ll post it there.