SFTP server encryption at rest using .NET Assembly

Is the file encryption feature for SFTP servers enabled by default or exposed in the .NET Assembly in any way? If not, do I need to do my own encryption to get the same result? I've looked through the documentation but haven't found anywhere to specify an encryption key.

https://winscp.net/eng/docs/file_encryption
https://winscp.net/eng/docs/ui_login_encryption

Use EncryptKeyPlain raw site settings:
https://winscp.net/eng/docs/rawsettings#encryptkeyplain

WinSCP GUI can generate a code template for you:
https://winscp.net/eng/docs/ui_generateurl#code