How can I force WP to use the www in the URL

Yeah… just add www. in the General Settings area in the admin!

This goes in the top of your .htaccess file:

RewriteEngine on
RewriteCond %{HTTP_HOST} !^www.your_domain.com$
RewriteRule ^(.*)$ https://www.your_domain.com/$1 [R=301]

Of course change the yourdomain.com part to your actual domain, also having both the www and non www can present a problem with Google, while the above should take care of that, you can also create a webmaster tools account, submit a sitemap and under site configuration > settings, set your preferred domain: https://www.google.com/webmasters/tools/ this will have otehr benefits also. ??

Thanks for all your help I will try your suggestions