It is not necessary to register a new domain name for shortening links if you own one that works properly. Using your domain registrar create a subdomain of the domain you already have, configure the necessary settings and then add the subdomain to Short.io as follows:
Sign in to your Short.io account.
Click on Add domain and then on Add own domain:
Enter your domain name and click on This is domain I already use for website or blog:
Select an option according to your subdomain name and click on Add domain.
Open the Domain settings and check if the subdomain is in status Active:
Important note: If the subdomain is Inactive then wait for a few hours and click on Refresh to check if the status has changed.
You will receive an email from the Short.io team informing you that the subdomain is now configured. Feel free to shorten your links and enjoy Short.io.