Show www-prefix on my blog

Hi,

I have a domain name and I have mapped that to my WordPress blog. Let’s say it is example.fi. It works fine, but I would like my blog to show www-prefix in front of the address: https://www.example.fi. Now when I type the https://www.example.fi to browser’s address bar, it is redirected to https://example.fi. How I can show also the www-prefix?

I googled and found the url address should be in Settings->General->WordPress address (URL)/Blog address (URL), but I do not see these options as they are visible only on self hosted WordPress installations.

Can anyone help me?