An M-***** code is an extra security measure initiated by Mercuryo to protect your funds. Mercuryo sends an M-***** code to your bank and your bank delivers a 5-digit M-***** code via:
Invoice from your bank's web/mobile app.
Some banks lack the technological means to deliver the code, unfortunately. If you haven't received it, please, contact your bank to find out if they have sent you the code or if there is an alternative way to receive the code and insert it in order to facilitate the transaction.