Send notification mail based on custom field value updated ?

[Cloud]

How to trigger notification mail to send only watchers 
depends on the custom field(Trigger) value is change to "Red_Flag" during all workflow transition status ?

I have to trigger the mail based on the custom field(Select list) value changes to "Red_Flag" during the transition,is it possible ?

How to trigger a mail based on custom field updated and customized mail content?

Please give me some suggestions,thank you!

[Tarun Sapra · Accepted solution]

Changing the custom field would trigger "Issue update" notification which is a generic notification. In order to make sure that the notification is because of the change of custom field, you would need to write custom listeners.

https://developer.atlassian.com/jiradev/jira-platform/guides/other/tutorial-writing-jira-event-listeners-with-the-atlassian-event-library

If you script runner plugin then it becomes even more easy

https://scriptrunner.adaptavist.com/5.0.0/jira/listeners.html

https://scriptrunner.adaptavist.com/5.0.0/jira/listeners.html#_custom_listeners 

sample 

https://community.atlassian.com/t5/Product-Apps-questions/Custom-listener-to-detect-User-Picker-field-change-and-copy/qaq-p/440758

[Cloud]

Is it possible  to send mail to only watchers group when custom field updated to "Red_Flag"?

I have already installed script runner plugin but I can't find the suitable built-in scripts.

Thanks for your great support.

[Vladimir Horev _Raley_]

Easy way to do that with our add-on Raley Email Notifications

Specifically, this article explain how to configure the Notification in details: 

Sending a notification when custom field value changes

Vlad | RaleyApps