You can include external JavaScript libraries or any other script in your Google Apps Script projects in multiple ways.

The best option would be create a new script (.gs) file inside your Google Script and copy-paste the entire JavaScript code into the file. This makes it easy for you to debug the code right inside the Apps Script IDE.

Alternatively, you can create a new HTML file inside apps script, copy-paste the code inside that file and use the eval() method as shown here:

function loadJSFromHTMLFile() {
  var javascript = HtmlService.createTemplateFromFile('script.html').getRawContent();
  eval(javascript);
}

If the JavaScript file is on a remote server or your Google Drive, you can use the UrlFetchApp and Drive.getFileById() method respectively to import the script into your Google Script at execution time.

// Load JavaScript from External Server
function loadJSFromServer() {
  var url = 'https://example.com/script.text';
  var javascript = UrlFetchApp.fetch(url).getContentText();
  eval(javascript);
}

// Load JavaScript from Google Drive
function loadJSFromGoogleDrive() {
  var rawJS = DriveApp.getFileById(id).getBlob().getDataAsString();
  eval(rawJS);
}

Finally, if you need to load multiple JavaScript libraries from a remote CDN, this technique by @BriaEgan will be useful. It creates the variables in the global namespace.

// Credit Brian @github
var LIBRARIES = {
  prettyDate: 'http://ejohn.org/files/pretty.js',
  underScore: 'http://underscorejs.org/underscore-min.js',
};

Object.keys(LIBRARIES).forEach(function (library) {
  newFunc = loadJSFromUrl(LIBRARIES[library]);
  eval('var ' + library + ' = ' + newFunc);
});

function loadJSFromUrl(url) {
  return eval(UrlFetchApp.fetch(url).getContentText());
}

eval() is found to be slower than the alternatives, since it has to invoke the JavaScript interpreter, while many other constructs are optimized by modern JS engines.