During authentication WinSCP will present you with authentication window to show you progress of authentication and to prompt you to enter your credentials or other information.
If you do not specify username on Login dialog, WinSCP will prompt you once it needs it for authentication.
If you are using password authentication, you can enter your password directly on Login dialog. Otherwise WinSCP will prompt you once a server asks for it. You will be prompted again, if you enter invalid password. Also note that most of the servers will "prompt" for password when you use invalid username, not to allow attacker to learn if particular account exists.
When authentication with stored password (if any) has failed before, the prompt will include Change stored password to this one check. Use it to overwrite stored password with new one.
For keyboard-interactive, TIS or Cryptocard authentication methodsthe server can issue its own prompt. Most of the servers use keyboard-interactive authentication just as different method for getting account password. For the reason WinSCP by default responds to the first keyboard-interactive prompt with the password entered on Login dialog (if any). To disable this behaviour unset keyboard-interactive authentication option Respond with password to the first prompt.
The server may also issue multiple prompts at once. Some servers will use such prompt to request a password change.
Private Key Passphrase
During authentication against SSH server the server can issue authentication banner. It can contain various information, generally regarding security and server usage. After reviewing the text, press Continue button to continue.
If you want to avoid seeing the banner each time you login, check Never show this banner again. It will make WinSCP ignore the same banner for the particular account and server the next time. However if the banner content changes you will see it again. Suppression of banners can be restricted by system administrator.
During authentication, the server may request you to change a password, particularly if your password has expired.
Enter your current password to Current password.
Enter your new password to New password and once again to Confirm new password.
Note that the server may use keyboard interactive method for change password request. Then the prompts will differ.