Hi, I have a question when adding my domain to digital ocean.

I've created a droplet called app, using one click dokku on Ubuntu installer, and have a bought a domain name, say, domain.com.

What I want to do is to host my app at the adrress: app.domain.com. And the question is, when I add domain.com for the first time, which droplet should I link it with? Is it just app, or do I need to create another droplet using Ubuntu for my domain? And what if I want to host another app at say app2.domain.com in the future?