curl --request POST \
  --url https://api.mpcvault.com/v1/rejectSigningRequest \
  --header 'Content-Type: application/json' \
  --header 'x-mtoken: <api-key>' \
  --data '
{
  "uuid": "<string>"
}
'

{
  "error": {
    "message": "<string>"
  }
}