Since the library was not loaded by the theme in my case I needed to do a form alter and then load the library. You can use something like this:
$form['#attached']['library'] = 'core/underscore';
If you are adding your own library will need to define that in ...
Defining the library is only half of the process. Libraries are not loaded on every page by default - you need to tell Drupal when it should include your library.
I bet you want to attach the ...