I have documented the AS/400 commands:
https://winscp.net/eng/docs/faq_mainframe

Thank you very much Martin! Looks like it is working:)

Use SessionOptions.AddRawSettings("PostLoginCommands", "site%20namefmt%201%0D%0Asite%20listfmt%201%0D%0Acwd%20/qsys.lib/tp01edi01.lib/edispstai.file")

See documentation:
https://winscp.net/eng/docs/library_sessionoptions
https://winscp.net/eng/docs/rawsettings

Does WinSCP .net dll support PostLoginCommands? Trying to ftp to AS/400 but that requires library/file/member pattern.
Or do I have to use ini-file somehow? Example with ini-file

[Sessions\xxxx]
HostName=xxxxx
PortNumber=21
UserName=my-userid
FSProtocol=5
LocalDirectory=C:%5CUsers%5Cxx%5CDocuments
UpdateDirectories=0
PostLoginCommands=site%20namefmt%201%0D%0Asite%20listfmt%201%0D%0Acwd%20/qsys.lib/tp01edi01.lib/edispstai.file
Password=my-encrypted-password