Using _user security variable inside ingest pipeline

Hello,

I have setup a domain that uses IAM auth via cognito. I created the below ingest pipeline. However, the user property is always an empty string. Does aws open search support the _user security variable.

PUT _ingest/pipeline/post
{
  "description": "post",
  "processors": [
    {
      "set": {
        "description": "Add user info",
        "field": "user",
        "value": "{{_user.username}}"
      }
    }
  ]
}