Mail user address from custom field

[Christian Radwanski]

Application: Jira Service desk (server)

Use case: reception fills in forms in service desk every time a visitor will visit our office.
Each time a visitor is reported, the visitor is sent files/map on how to reach our office and some other formalities.

Goal:
A custom field contains the mail-address of the visitor. Make Jira service desk utilises this field to send a notification to the user upon closing ticket. The message will contain a sharepoint link with files for the user.

Tried:
Tried to look into creating a custom mail listener, preferably I would like to find the file responsible for the main mail listener and use that as an example (com.atlassian.jira.event.listeners.mail.MailListener).

Checked documentation:
https://confluence.atlassian.com/adminjiraserver073/listeners-867028226.html?_ga=2.32268606.124229158.1581273227-999996041.1519045498

Remarks:
Unfortunately I does not look like I will learn creating a custom listener in one week, hopefully there is another way of achieving my goal. Tried finding an example custom listener to speed up the learning process but did not find one online.

We are not interested in plugins other then scriptrunner but again, it will take time to master scripting in scriptrunner. Is someone maybe willing to share an example/similar script/custom listener?.

Any advice on how to approach/proceed with my issue would be much appreciated.