You can customize the URL when you’re in the process of creating a new link. To change the subdomain before lnk.to go the ‘Link’ section and click the arrow. You can choose between previously used domains or create a new domain. To create a new domain simply type in the domain you wish to use and it will be ready for you to use.
At this step it's also possible to customize the short code which is the part coming after lnk.to. Simply delete the characters already there and type in your preferred short code like for example the name of the single, album or artist to make a link looking like this artist.lnk.to/albumname - see screenshot below