ssh to remote server and switch user

Hi all,

I have installed sshpass.

I am using sshpass and ssh to login to remote server.
Once I log into to the remote server I would like to switch to webapp user for which I would like to pass the password from the same command line where I initiated the sshpass on my terminal.

example:

sshpass -f passfile ssh myself@abc.com 'su - webapp'

The above command cnnects to abc.com host and asks for password and when I type the password is visible what I type, and fails to switch to webapp user.

Can someone help?

Thanks,
Ram.