Banks let you add a message or reference to the receiver when initiating a transaction. Usually this is in the form of a field called "Message to receiver", "Reference", "Originator to Beneficiary Information (OBI)", "Remittance Information" or similar.
Some banks offer multiple reference fields. When this is the case, one might be a personal internal reference only, so it would not be sent in the transfer to our bank. If you are not certain about how to attach a reference to your bank transfer so that it is sent to the receiving bank, contact your bank for help.