Welcome! I'm moving your post to the General Discussion area so that more members will see it and reply.
What you want to do is possible using URL re-directs where you point a URL to the specific address for each FileMaker WebDirect app. I can't help you with setting up those redirects, though.
I suspect you might run into issues with SSL certificates if they are completely separate domains. If the different websites are simply separate sub-domains, you could get a wildcard SSL certificate.
I think you can specify the database name to open.
The syntax is fmp://<YourServer>/<DBfile>
Something like this: fmp:/acme.com/Main%20Menu.fmp12
I have read some post that if the %20 does not work (this is a space) try a plus sign.
Even if we are accessing it via https and not via fmp?