Use a domain or subdomain that you control. Point it to the same AdLinkFly installation, and configure the host and HTTPS. Then add or edit it under Admin > Settings > Domains, and set Default Short Domain to Yes.
New short links that use the default will use this hostname. If you do not select a default short domain, AdLinkFly uses the main domain. A system domain with both Main Domain and Default Short Domain set to No stays available. Eligible members can still choose it as an extra short domain.