When you create a database and database user in cPanel, it prepends your cPanel username to each.
For examply - say your cPanel username is ‘domacom’.
Now, you create a database named ‘magento’ - the actual database name is ‘domacom_magento’ - there is no prefix here - this is your full database name. The same thing is true for the username.
If you create a database user in cPanel named ‘dbuser’ - the actual database username would be ‘domacom_dbuser’ - again, there is no prefix, this if your full database username.
So, when installing Magento it asks for database name, that would be ‘domacom_magento’ and the database username would be ‘domacom_dbuser’. You would not put anything in the prefix field unless you are using database table prefixes, which you most likely are not.
Hope this helps.