First, it’s important to distinguish between two terms used in Steam: login and profile name. The key difference is that the profile name can be changed, while the login always remains permanent.
Login is a unique user identifier used to log in to the system along with a password.
Profile name is your public nickname, visible to other users in Steam.
Keep in mind that when topping up your Steam account, you must specify your login, not your profile name.
To find out your Steam username, follow these steps:
1. Log in to the Steam program on your PC or via the website.
2. Click on your username, located in the top-right corner.
3. In the menu that appears, select the "Account Details" tab.
4. On the page that opens, after the word "ACCOUNT", you will see your login. Copy it.