How to make a domain point to a specific port for a Minecraft server?

You can't. *

Domain names don't support that. They're merely a human readable alias of one or more IP addresses and cannot be linked to a specific port.

If you want your players to be able to just use the domain name without port you'll need to use the default port (for Minecraft it is 25565). The default port will always be used when a player doesn't specify a port in the game. For example: mc.example.com:25565 = mc.example.com.

Or if you have access to it you could as well redirect the default port to yours. For example with iptables. But that would only work if the default port is not in use (e.g. by another Minecraft server).

* Some providers do allow it in their control panel as a part of unmasked redirect. They then redirect the traffic from that domain using their local settings to the target ip:port. This is no common practice so I cannot tell who actually supports it.