mention someone on slack

[Hadas Hamerovv]

When trying to mention someone in slack (or send him DM) it works when using the field assignee

@{{issue.assignee.properties.metadata.slack_id}}

However when the same information is stored under a custom field (user picker) it does not work.

@{{issue.customfield_10043.properties.metadata.slack_id}}

Does someone knows the exact syntax or what is the reason?