No, FileMaker does not allow manipulating the URL in WebDirect to log in as part of the security controls.
The solution is to have a user auto log in as a guest and do whatever you want a guest to be able to do. You can also give them a button to relogin if they want a higher level of security.
Auto loggin in as a guest is a good way to have a web page anonymously for things like feedback on a web page or displaying some data from FileMaker publicly. You just have to control behaviors with your startup script and make sure the guest account doesn't have access to tables or layouts that it shouldn't have access too.