How to write an advanced function that can append users in the Multiple Assignees field?

[Maham Nadeem]

• Automation Rule I want to enable: When the assignee field is changed, the last assignee should be added to the Multiple Assignees field on the issue. 
• If I try to implement this using smart values, the last assignee replaces any value in the field instead of appending the new value in the field.
• I have tried to achieve this using advanced functions. The audit log shows SUCCESS but the function does not do anything i.e the Multiple Assignees field is not updated when the Assignee on the ticket is changed. 
• Multiple Assignees field is a multi-select user field. 
• 

[Trudy Claspill · Accepted solution]

Hello @Maham Nadeem 

Welcome to the Atlassian community!

This is the JSON that you need:

{
  "update":{
    "Multiple assignees":[
      {
        "add":{
          "id":"{{changelog.assignee.from}}"
        }
      }
    ]
  }
}

[Bill Sheboy]

Hi @Maham Nadeem -- Welcome to the Atlassian Community!

Adding to Trudy's answer, here is the how-to documentation for making a JSON edit like that for multiple-user fields:

https://support.atlassian.com/cloud-automation/docs/advanced-field-editing-using-json/#Multi-user-picker-custom-field

Kind regards,
Bill

[Maham Nadeem]

It works! 

The heartiest thank you @Trudy Claspill @Bill Sheboy
Both of you have been the most kind! 

Grateful to be part of this community.