Before Add domain on your Plesk Web Admin, if you are pointing to free dns zone as per below in that case you may need to add and update the DNS by referring this URL (here) before proceeding add the account.
Here will show you the step how to create an account from your Plesk Web Admin.
2. Click on "Add Domain" under Websites & Domains
3. After that fill in the domain detail and webspace settings as per below.
4. Once done above, click back to Websites & Domains it will show your domain name there and now you can manage your domain features.