curl --request POST \
  --url https://mainnet.gomaestro-api.org/v1/contracts/subscription/createUserAccount \
  --header 'Content-Type: application/json' \
  --header 'api-key: <api-key>' \
  --data '{
  "subscriber_address": "<string>",
  "selected_out_ref": {
    "tx_hash": "<string>",
    "output_index": 123
  },
  "email": "<string>",
  "phone": "<string>"
}'

{
  "cbor_hex": "<string>",
  "tx_hash": "<string>"
}