If we want to add a custom css to the input-group-addon:

<div class="input-group"><input type="text" name="example2"><span
class="input-group-addon newCss1 newCss2">@domain.com</span></div>

Using something like this:

$input->afterAddon('@domain.com')->addAddonCss('newCss1')->addAddonCss('
newCss2');