Custom field for Jira Cloud

{
  "text":"<eventType> <issueLink> in <projectLink> by <userLink>",
  "attachments": [
    {
      "color": "#205081",
      "text": "<eventMessageBody>",
      "fields": [
        {
          "title": "Type",
          "value": "<issueType>",
          "short": true
        },
        {
          "title": "Assignee",
          "value": "<issueAssignee>",
          "short": true
        },
        {
          "title": "Creator",
          "value": "<issueCreator>",
          "short": true
        },
        {
          "title": "Custom Filed",
          "value": "<issue.fields.customfield_10009>",
          "short": true
        }
      ]
    }
  ]
}