These steps must be carried out from the Plesk control panel:
1- Enter your Plesk panel from:
https://www.yourdomain.com:8443
Place your access information there.
2- At the top, click on "Add Domain Aliases".
3- Then, where it says: "Name of the domain alias" we place the domain name that you want to add.
Then in the lower part we tick all the check boxes and press "accept".
4- Now your new domain should appear in the section "Websites and domains" in the Plesk.
There you can assign a folder of your web space and create the mail accounts you want.