Usually, VPN software does not allow as input the password for a user, because it is considered a security risk.
A possible solution is feeding the password via a pipe as in:
echo YourPassword-e "Tunnel all\nYourPassword" | openconnect --authenticate --user=<username> "VPN host"
If we are talking about you being interested in this method to write a script:
-  be sure to understand the security implications of having your password in a file, and restrict the read rights of that file only to the user running the openconnectcommand.
PS Replace YourPassword with your real password