When a user adds EVE Online to their game library through the Steam store, Steam creates an EVE Online account for the user. An account created through Steam is considered a "Steam integrated account".
When EVE Online is launched from the Steam game library, the user will be automatically logged into the EVE Online account associated with their Steam user account - there is no need to enter a username or a password.
By default, Steam integrated EVE Online accounts do not have traditional usernames & passwords. To log into these accounts with the standard EVE launcher without utilizing Steam, the user first needs to create a unique username and password. Please see the following article for instructions on this:
Special Steam offers available in the Steam store are only applicable to the Steam integrated accounts. They cannot be purchased & applied to standard EVE accounts created directly from the EVE Online webpage.
Steam wallet however can be used as a payment method for any account, please see the following article for details: