AdministratorPassword is encoded incorrectly

[HeinziAT]

Repro (private information redacted):

var options = new AnswerFileOptions()
{
    InstallOptions = new CleanEfiOptions()
    {
        ImageIndex = 2, // Windows Server 2025 Standard (Desktop Experience)
    },
    ProductKey = ...
    EnableRemoteDesktop = true,
    TimeZone = "W. Europe Standard Time",
    ComputerName = ...,
    AdministratorPassword = ...,
    EnableServerManager = false,
};

AnswerFileGenerator.Generate(@"...\autounattend.xml", options);

This will fail with the following error message: "Windows could not complete the installation. To install Windows on this computer, restart the installation". setupact.log shows the cause:

2026-04-22 23:03:32, Warning                      [Shell Unattend] Failed to decode password (0x8007000d) 
2026-04-22 23:03:32, Info                         [Shell Unattend] Exiting 'oobeSystem' pass with status 0x8007000d

The reason for that seems to be that AnswerFileGenerator.cs encodes the administrator password as <password>Password, even though it needs to be encoded as <password>AdministratorPassword.

                Writer.WriteStartElement("AdministratorPassword");
                Writer.WriteElementString("Value", Convert.ToBase64String(Encoding.Unicode.GetBytes(Options.AdministratorPassword + "Password")));

(Unfortunately, I cannot provide a link to an official source explaining that this particular suffix is needed, since the official documentation does not mention the suffix at all. Claude Opus suggested that I replace the base64 encoded <password>Password with <password>AdministratorPassword in the autounattend.xml file, and it fixed the problem for me.)