You generally only need to convert to a Username and Password if:
To address your request, it is important to clarify that You cannot convert a modern 20-character License Key into a Username and Password. Instead, the process works in reverse: you must convert legacy Username and Password credentials into a License Key to activate newer versions of ESET products . How to Convert Legacy Credentials to a License Key
After reviewing all methods, the to convert an ESET license key to a username and password is: