How do I set a user property (ei: reporter, set his slack ID) and then use it at Automation for JIRA

[Joaquín Jadue]

I'm trying to set a User property:

key: slack

value: 12345

How can I get to use it using smart values? For example, at Automation for JIRA for sending a slack message?

We are using JIRA cloud.

BTW, I can't find the "Add-ons" section User properties editor.