Did you check the "Remember session password and pass it to PuTTY"?
Also make sure you upgrade to 5.9.1. It has some related fixes.
Using WinSCP 5.9.
I am trying to use ctrl+p from WinSCP to open my ssh client.
Under Preferences | Integration I am using this command line:
"C:\Program Files\VanDyke Software\SecureCRT\SecureCRT.exe" !@ /L !U /PASSWORD !P
But the !P does not add the password. It is empty.
And the password is stored with the session, double click the session in WinSCP logs into the server without any problem.