If this is for internal only you can setup OpnSense to do DNS records either through static IPs or add hostnames to the DNS records via DHCP. If you want this externally that’s a whole other ball of wax.
Neither require you to purchase a domain name. Just use whatever you want .internal While you can use other things like .lan, .internal is the only one that is actually reserved for LAN use.
*I’m pretty sure you can add firewall rules in OpnSense as well to redirect from port 80 or 443 to whatever port you’re actually using but that may cause more problems than it’s worth. If you can’t change the port in the app itself then, honestly just bookmark it with the actual port so you don’t have to remember it.