curl --request PATCH \
  --url https://app.herondata.io/api/merchants/{merchant_heron_id}/user_merchant_group \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '
{
  "group_id": "debt.mca_or_rbf"
}
'

{
  "merchant_heron_id": "<string>",
  "merchant_name": "<string>",
  "group_id": "<string>"
}