Go to your domain provider and add an 'a record' for your subdomain. In my case I am using Namecheap. I had to create an 'a record' with * as the Host and my server's Ip as the value.
Then in the deploy.yml of Kamal do the following, go to proxy:, and where it says host: just type in the name of the complete subdomain. Example: host: store.mydomain.com. This has to be the same as what you used in your DNS provider.
Then commit that and deploy it running bin/kamal deploy.
If that works after deploying that's great. I've heard that updating the DNS may take some time to take effect too.
After deploying with Kamal I noticed that the proxy hadn't been modified and that caught my attention. I executed bin/kamal proxy reboot and that re created (or restarted) the docker container for the proxy and my subdomain was then working.
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