It looks like it is not possible to use username "scp" to automate login.
C:\Temp>WinSCP.exe /console /command "open scp:firstname.lastname@example.org"
gives next result:
Searching for host...
Connecting to host...
Using username "password".
Using keyboard-interactive authentication.
Username scp is not taken into consideration but "password" is used instead. Is there any way to automate login for username "scp"?
OS Name: Microsoft(R) Windows(R) Server 2003, Enterprise Edition
OS Version: 5.2.3790 Service Pack 1 Build 3790