If you have Apache server in front of Confluence, create Apache virtual host for each virtual host handled by Domain Booster in Confluence. For example:
Virtual host: first.domain.com
<VirtualHost *:80>
ServerName first.domain.com
HostnameLookups off
ProxyRequests off
ProxyPreserveHost On
ProxyPass / http://first.domain.com:8090/ timeout=35
ProxyPassReverse / http://first.domain.com:8090/
<Proxy *>
Order allow,deny
Allow from all
</Proxy>
</VirtualHost>
Virtual host: second.domain2.com
<VirtualHost *:80>
ServerName second.domain2.com
ServerAlias third.domain2.com
HostnameLookups off
ProxyRequests off
ProxyPreserveHost On
ProxyPass / http://first.domain.com:8090/ timeout=35
ProxyPassReverse / http://first.domain.com:8090/
<Proxy *>
Order allow,deny
Allow from all
</Proxy>
</VirtualHost>
The server base URL in Confluence configuration could be set to first.domain.com.