Custom domain support for users with Node and Express

I want to offer support for custom domains for my users. I created a file in /etc/nginx/sites-avilable/ with a server block that routes everything to an Express app (running with PM2). I want to use Express to get the domain they are requesting from, cross reference that domain name, and show the appropriate user’s page. However, something like req.headers.host only gives me ‘localhost:3000’. How can I get the domain name of the request? Or is there a simpler way of doing this?