curl --request POST \
  --url https://whitebit.com/api/v4/main-account/smart-flex/investments/close \
  --header 'Content-Type: application/json' \
  --header 'X-TXC-APIKEY: <api-key>' \
  --data '
{
  "plan": "8f2e9d3c-1a4b-4c2d-9e5f-6a7b8c9d0e1f",
  "request": "{{request}}",
  "nonce": "{{nonce}}"
}
'

{
  "data": {
    "transactionId": "tx_9f3e0d4c-2b5c-4d3e-8f6g-7a8b9c0d1e2f"
  }
}