I have just recently switched from SuperGenPass to LastPass because the master password cannot be changed with SuperGenPass.
Now with SuperGenPass to would type in your usename and master password in the login screen and active the SuperGenPass script bookmarklet, which would replace the master password with the correct (generated one). This is the behaviour that I want. No autocomplete of either username or password. When you've filled in both fields to just click the bookmark (or a short-key would be best) and LastPass would take over and fill in the correct password.
Is this possible with LastPass and how would I do that?