Yes, it is possible.
However, before you begin, please make sure you have followed the instructions on How to set up a domain for short links.
To add www to your domain
Go to your domain registrar and add a CNAME record as follows:
Host: www
Type: CNAME
Value: cname.short.io
Important note: If the domain is Inactive in Short.io then wait for a few hours and click on Refresh to check if the status has changed.
Hereafter, the links you shorten using this domain can be accessed in one of two ways: either with or without www at the beginning.