If you opt to default to the www prefix, all requests for yourdomain.com will redirect to www.yourdomain.com using the search engine friendly 301 redirect. The opposite is true if you opt to default to NOT using the www prefix.
You can find this option in Plesk by clicking on the "Hosting Settings" link beside the domain you wish to configure. Look for the "Preferred Domain" option and set it to your preferred option.
Warning: if you set the option in Plesk to the opposite of how your "Site URL" is configured in WordPress (or similar settings in other web apps) you will create a redirect loop where the web server is pointing to one option and your web app then attempts to load the opposite. Be sure that your preference of using the www or not using the www matches in Plesk and your web application of choice.
If you are using Plesk 18.x here's how to fix this:
- Log in to Plesk
- Find the domain experiencing the issue
- Click on the "Hosting Settings" for the domain (you may need to expand the domain's view)
- Ensure the preferred option is selected (WWW or non-WWW)
When adding a domain in Plesk, it provides a quick setup process where this option appears. Unfortunately it can be easily skipped (by clicking finish rather than next) or overlooked. Remember to read through the options carefully when adding a domain in Plesk.