You can use the `addClass()` method of the attribute class:

    {%
      set classes = [
        'my-class-1',
        'my-class-2',
      ]
    %}

    <input{{ attributes.addClass(classes) }}>

You can either pass an array like the example above or pass a single class in the method directly: `attributes.addClass('my-class')`. 

For reference see the Drupal docs: https://www.drupal.org/docs/8/theming-drupal-8/using-attributes-in-templates