In your example you are telling it to place the transaction id as a GET request in the URI under the tx parameter.

walletnotify=curl http://My.IP.Address:PortAppUses/walletnotify.js?tx=%s

All you would need to do inside of walletnotify.js is pull the tx GET, in nodejs express that would be something like:

const express = require("express");
const app = express();
const request = require("request");

app.get("/", function(req, res){

    let transactionId = req.query.tx;

    //do whatever with transactionId

});