If you have a hosting plan in which you can host more than single domain but you don't know the way, then you can check it out:
To add new domain in your Plesk account:
first, you have to change the nameservers of your domain at your domain registrar to :
Nameserver 1
Nameserver 2
then, login to Plesk >> click on "Add domain" option
Here you can create your addon domain in Plesk.