When you register a domain name, what you register is called the root domain, e.g. iwmn.com. Some services require you to set up your domain with the "www" prefix: www.iwmn.com. That's fine, you don't have to register or separately set up anything to do that.
This isn't a separate domain name that you would register. (It's not actually possible to register a domain with the "www." at the front.) You would just include the "www" prefix when you set up the domain with your hosting platform or service. (Zenfolio is a common example of this setup requirement.)
The root domain is also what you use to create custom email addresses. When you set up your email hosting, you create usernames to complete the email address. E.g. if you wanted a general email address for your company, you might use "info" as the username, so the email address would be email@example.com.
Or if you are setting up a personal account, you could use your name and create something like firstname.lastname@example.org or email@example.com.
More information here about setting up custom email hosting and addresses.
When setting up email, you set up nameservers or DNS records (to connect the domain name to the hosting provider) in your iwantmyname dashboard, but you would not create the email addresses there. That happens in the email hosting account you set up.
To offer a broader range of options and services, we have a variety of email hosting provider partners. You can see the full list in our Marketplace.
Many people are just looking for a simple custom email option and one or two addresses. Zoho Mail can be a good choice for them, as we have a full setup guide, and it remains the only hosting provider with a free account option. (Though the free plan has more limited features, so definitely read up on what's included to make an informed decision.)