Login to Plesk at https://plesk.colony1.net:8443/
Choose Domains and then click the specific Domain.

If your current Primary Domain is not being used any longer you can Rename Domain.
To Rename Domain, under Domain Administration click the down arrow and click the choice for Rename Domain.

If you want to add additional Domains that point to the same Primary Domain you can add them under Domain Aliases.
To add Domain Aliaseses, under Website click Domain Aliases.

Note: When adding new domains make sure to point the name servers to DNS4.Colony1.Net and DNS5.Colony1.Net.  This can be done at the location where you registered/purchased the Domain.