There is no need to use account names/passwords as this is taken care of by FileMaker ... just point the user to the database, <https://yourURL/fmi/webd#yourDatabase> and whatever login you have set up in FileMaker will be the default login for WebDirect. e.g. if you have set up to "auto login" using a user/password (File -> File Options...) this will be used to log into Web Direct. Conversely if you have set the database to require a username/password, the user will be challenged.
There is no way to pass it through a URL as far as I know. It's a huge security risk, and I'm sure it's left out on purpose.
You can have a default login (IE guest), and a re-login script if you need a user to log in with a specific account.
Do you mean to alter the URL with account=guest sort of thing?
If so, I look for this as well. I had a working login form that forwarded Account and Password to an IWP solution. Now, I can't get it to work with WebDirect...
Not a guest account, but it's probably the same tweak.