how to include primary site url with www in multisite setup?

You need to alter the siteurl and home option settings, but they are not accessible any longer once you configure your multisite. At least not through the usual network or main site UI. There’s a semi-hidden options page where you can update these settings to include the www in the URLs. Go to /wp-admin/options.php on your site by typing into your browser’s address bar. Search the extensive list for “home” and “siteurl” settings. Add in the www. for each option, then save using the button at the bottom of a very long page.

This will cause the main site’s dynamically generated URLs to include the www, but any static references in content, such as image file references, will still be without the www. You need to seek out all domain name references in the DB and update accordingly. To do so, you need a tool that knows how to deal with serialized data. I recommend the inter.connect tool.