Currently this is not possible per the Restricted functions in JIRA Cloud applications knowledge base article:
To customize Jira notifications, one must edit Velocity files within the Jira web app.
This makes it a special case of "customizing source files". See the Suggestion at CLOUD-1791 - Customize notification email template Closed and the Suggestion at JRASERVER-7266 - Create an Email Template Editor in the UI to Replace Editing Velocity Templates.
At this time CLOUD-1791 is closed but you can still vote on JRASERVER-7266 and add your comments for the developers.
That doesn't help as that is for JIRA Server whereas the corresponding Cloud ticket (marked as resolved) states;
"In the spirit of transparency, we are going to close this feature request for JIRA Cloud. It's been 18 months since we last provided an update and we do not have any plans to introduce support for custom email templates in JIRA Core or JIRA Software. (This is how we've been interpreting this feature request, as editing velocity templates has never been possible for JIRA Cloud). If you are a JIRA Server customer, please continue watching JRASERVER-7266 for updates."
Which, to me is a lazy and not a customer-focused response - we're closing the ticket because we haven't bothered to update it in 18 months, we've been interpreting the ticket how we please and haven't requested clarification from users and because this has never been a feature of JIRA previously, we aren't adding it.
The fact it hasn't been closed recently means that assumption is incorrect. Atlassian started an initiative to review all open requests in their Jira a few months ago, and this one survived the initial run of "close if we're sure we won't do it"
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