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