DNS Help needed

Assume:

- my WAN IP is xx.xx.xx.xx

- I have bought a domain mydomain.com

I have so far successfully set up dyndns on my home pfsense.

On pfsense I have got port forwarding configured for my plex server to be able to be reached over web at xx.xx.xx.xx:32400 and this is working fine when being accessed with PublicIP:port

Now I want to have a record added as plex.mydomain.com which should automatically point to port 32400 (probably it should automatically redirect to plex.mydomain.com:32400)

How to do that?

P.S. - DNS is being managed in Cloudlfare.