curl --request POST \
  --url https://api.ara.so/admin/users/{user_id}/plan \
  --header 'Content-Type: application/json' \
  --data '
{
  "plan": "<string>"
}
'

{
  "detail": [
    {
      "loc": [
        "<string>"
      ],
      "msg": "<string>",
      "type": "<string>",
      "input": "<unknown>",
      "ctx": {}
    }
  ]
}