Its a mystery to me, why it is so hard to create a wild card sub domain in plesk. Any way here is how it is done.

First go to Domains>domain.com>Dns Settings
Then go to “Add record” and add the following record

Record type: “A”
Domain Name: *.domain.com
IP Address: ip address of the main server (us usually your plesk address ip)

After that restart your domain name service.

Now you will need your root user info to do the rest of the job.

Log into your server using root info (not in plesk). Then go to
/var/www/vhosts/domain.com/conf
Here check if you already have “vhost.conf” then edit that otherwise
create a new one using following command

vi vhost.conf

then add the following entry in the file

ServerAlias *.domain.com

Now save the file and get back to plesk and restart your apache and dns(bind) service.

Give it some a little time to get updated and then try.