Add a custom script to subtheme

Hi @drupalfan2 →
step 1 - In your sub-theme folder in custom_theme.libraries.yml file paste below code, this create a link of main.js: in scripts folder JS file .

#global custom
global:
  version: 8.x-3.1
  js:
    scripts/main.js: {}

step 2 - In your sub-theme folder in custom_theme.info.yml file paste below code, that will connect global libraries with your theme.

# custom library 
libraries:
  - custom_theme/global

step 3 - In your sub-theme folder in scripts folder create a file with name main.js.
you may testing by pasting the code.

/**
 * @file main.js 
 */

// Attach all behaviors.
(function($, Drupal, once, drupalSettings) {
  "use strict";
  Drupal.behaviors.global = {
    attach: function(context, settings) {
      // Attach first behaviors - you may bind the behavior on any element like 'html/body'
      once('myCustomBehavior', 'html', context).forEach(function(element) {
        console.log('first behavior ready');
      });
      // Attach second behaviors - you may bind the behavior on any element like 'html/body'
      once('myCustomBehavior_2', 'body', context).forEach(function(element) {
        console.log('second behavior ready');
      });
    }
  };
})(jQuery, Drupal, once, drupalSettings);

Note :
change custom_theme name with sub-theme name.