Use a domain or subdomain you control, point it to the same AdLinkFly installation, 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 fallback will use this hostname. If no default short domain is selected, AdLinkFly falls back to the main domain. A system domain with both Main Domain and Default Short Domain set to No remains an additional selectable short domain for eligible members.