Here is my solution.  I've made comments wherever I made additions to the tutorial code.
/*** index.js ***/

var express = require('express');
var router = express.Router();
var querystring = require('querystring');    // load the querystring module

// create a local variable
var videodata = require('../videodata.json');

var addy = 'https://www.thenewboston.com/videos_food.php?';    // variable for the base address

/* GET home page. */
router.get('/', function(req, res, next) {
 res.render('index', {
   title: 'Express',
   videodata: videodata,
   addy: addy,                      // add the additional variables
   querystring: querystring         // to the router.get method
 });
});

module.exports = router;

/*** index.ejs ***/

<!DOCTYPE html>
<html>
  <head>
    <title><%= title %></title>
    <link rel='stylesheet' href='/stylesheets/style.css' />
  </head>
  <body>
    <% include templates/header.ejs %>
    <h3><%= videodata.categoryName %></h3>
    <ul>
      <% videodata.categories.forEach(function (item) { %>
      <!-- Concatenate base address and query string to build URL -->
        <li><a href="<%= addy + querystring.stringify({'cat': item.categoryID}) %>"><%= item.categoryName %></a></li>
      <% }); %>
    </ul>
  </body>
</html>


This works but I couldn't figure out how to get the '?' into the URL without making it part of the base address string.
Does anyone have any ideas about this?