I have no experience with PowerBuilder. In any case, you need to use WinSCPnet.dll. Not DragExt64.dll. And to register WinSCPnet.dll for COM (if that's what PowerBuilder uses), you need to use RegAsm:
https://winscp.net/eng/docs/library_install#registering
Though quick google search reveals that PowerBuilder should be able to use .NET assemblies directly, without any registrations:
https://docs.appeon.com/pb2019r2/whats_new/ch01s03.html

HI Team,

I have installed winscp 5.17.10 and unable to connect the winscp from power builder application using class name WinSCP.Session.

So could you please share me the class name and kindly let me know whether we need to register WinSCPnet.dll or DragExt64.dll in my machine and I was unable to register the WinSCPnet.dll using regsvr32.

Thanks and regards,
Santhosh S
santhosh.somu@genre.com