In your control panel you need to explicitly add www.yourdomain.com as a domain name for your application that contains the redirect logic, and then you need to ensure you have the proper logic set up in your redirect application to direct people where they need to go. There's a couple ways to go about this, but here's what I do personally.

I create a dedicated application within the WebFaction control panel to handle all my redirects and nothing else. This should be a static/php application, and the only file it needs is a .htaccess file with something like the following:

RewriteEngine On

# ----- Don't redirect .well-known (required for letsEncrypt) -----
RewriteRule ^\.well-known - [L,NC]

# ----- Redirect HTTP -> HTTPS and remove www. -----
RewriteCond %{HTTP_HOST} ^www\. [OR]
RewriteCond %{HTTPS} off
RewriteCond http%1://%{HTTP_HOST} ^(https?://)(www\.)?(.+)$
RewriteRule ^ https://%3%{REQUEST_URI} [R=301,L]

# ---- If Nothing Above Matches, show a blank 404 -----
RewriteRule ^ - [L,R=404]

I also usually have other redirects in this file as well, but I've omitted them for brevity.

Then in your control panel, make this an http website, and add www.yourwebsite.com and yourwebsite.com as the domain names. You could also create an additional website that is https with www.yourwebsite.com using the same redirect application.

After that you just have your application set up like normal, whatever it may be, using https, and yourwebsite.com as the domain name without the www. prefix, without your application being blissfully unaware of any redirect logic.

What the code above does:

It first ignores any redirect rules for anything that begins with anydomain.com/.well-known/, as that is used by LetsEncrypt to verify your website's identity.

It then checks if the url supplied starts with www. or is http, and if so, will redirect the user to the https://www. version of the same URL.

Hope that all makes sense, if you're having any further issues I can try to help.