curl --request POST \
  --url https://api.multchats.com/v2/inbox/upsert-custom-fields-by-phone-number \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "phoneNumber": "<string>",
  "inputs": [
    {
      "templateFieldId": "<string>",
      "valueJson": "<unknown>"
    }
  ]
}
'

{
  "success": true,
  "response": {
    "customFields": "<array>"
  }
}