How to enter the address to the main domain in node js?

Question

Node.js

how to enter the address to the main domain of the site, for example <b>dop</b>.maindomain.com
, how to write this dop in the application on the Internet did not find
something like this?
app.use('dop./', routes.dop); - I use a separate folder with routes

Answer 1

req.subdomains