curl --request PUT \
  --url https://api.devhelm.io/api/v1/monitors/{monitorId}/assertions/{assertionId} \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "config": {
    "substring": "<string>"
  }
}
'

{
  "data": {
    "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "monitorId": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "config": {
      "substring": "<string>"
    }
  }
}