Multi-tenancy application

please I have to create a multi-tenancy application using node js(Sails js framework) and Angular, each client has his own username, by this username the client can get access to the application by typing username.myapp.com, the server should know the username to give me a port to associate it with the client (each client has one port to connect with the database). my question is how I can distribute a unique port for each tenant (username) using Nginx? Note that for new clients we should create a new port, for old clients we should connect with corresponding port