Firstly where it says user a record with name ‘dynamic’, that assumes you want to access the IP on the name ‘dynamic.yourdomain.com’. If you wanted to use the name ‘home.yourdomain.com’ then you’d natrually want to use a record name of 'home.
Secondly the value you put in for an A record doesn’t really matter - it will be updated to your dynamic IP by DNS-O-Matic when it next runs to mtch your home WAN IP address. You could either put in your current WAN IP (from whatismyip.com, say) to have it work right away, or put in something completely different and run DNS-O-Matic so you have the pleasure of seeing it update to your correct home IP. Up to you.
For info, back when I used DNS-O-Matic it frequently failed because they used to hit the limit of calls Cloudflare used to allow them. If you get this problem then you should know it’s pretty easy to just update Cloudflare directly using their API and there are plenty of scripts just a Google away which will do this for you.