You can customize the URL when you’re in the process of creating a new link or by "editing" your link later. To change the subdomain.lnk.to when creating a link go the ‘Link’ section and click the dropdown. 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 if it's not used by another account it will be added to your link.
At this step it's also possible to customize the short code which is the part after lnk.to/shortcode. Simply delete the characters already in that box and type in your preferred short code, for example the name of the single, album, or artist to make a link looking like this artist.lnk.to/albumname - see screenshot below