curl --request PUT \
  --url https://api.example.com/feature-flags/assignments \
  --header 'Content-Type: application/json' \
  --data '
{
  "flagKey": "<string>",
  "subscriberId": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "value": true
}
'

{
  "data": {
    "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "flagId": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "subscriberId": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "value": true
  }
}