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To create a custom block, add the following function to your module's object:

/**
 * Defines blocks for my_module.
 */
my_module.blocks = function() {
  var blocks = {};
  
  blocks['my_module_custom_block'] = {
    build: function () {
      return new Promise(function(ok, err) {
        var element = {};
        element['my_markup'] = {
          markup: '<p>Hello World</p>'
        };
        ok(element);
      });
    }
  };
  
  return blocks;
};

Next, if we add the block to a region the settings.js file, we'll be able to see the custom block.