Log in to your account on your registrar's website.
Go to the management of your domain name. Look for a section titled something similar to "DNS", "DNS Zone", "Zone File", "Manage Zone", "DNS Configuration", or "DNS Management".
Inside the DNS zone editor, you will need to create (or modify, if it already exists) a CNAME type record.
For this record, use as Alias (or Subdomain or Name) the value www.
The Value (or Destination) of this record must be d.artfolio.com.
Note: Access to the option to add a CNAME record may require switching to "advanced mode" or "expert mode".
Check if your domain name automatically redirects visitors from https://yourdomain.ext to https://www.yourdomain.ext. If this is not the case, you need to set up this redirection.
In the client interface of your registrar, look for a section titled "Redirects", "Web Redirects", or "Forward". Prefer to set up a visible and permanent redirection (301) rather than a transparent or masked redirection.
Need help?
If you are having difficulty setting up your domain name, contact your registrar. Indicate that you need to create a CNAME record for www pointing to d.artfolio.com, and that your address https://yourdomain.ext should be redirected to https://www.yourdomain.ext.