How do I set up dynmap with a custom domain?

use reverse proxy like nginx to do that

I used a subdomain to forward to the ip:port site. Then, add an A Record that points that same subdomain to your ip. It's a little bit of a workaround but it works for me!

iangioffre how exactly did you that? I mean what did you put in Host weight and prio

@alex17lol An A record shouldn't require weight and priority. I used GoDaddy and I was able to easily setup a subdomain in the forwarding tab. Then I add an A record with the same subdomain name with my ip and a TTL of 1/2 hour.