Create Simple Custom Field

#/themes/e_shop/views/custom_field/_country_select.html.erb
<div class="group-input-fields-content">
    <%= select_tag "#{field_name}[#{field.slug}][values][]", options_for_select(ISO3166::Country.all_names_with_codes), class: "form-control input-value #{"required" if field.options[:required].to_s.to_bool}" %>
</div>

// /themes/e_shop/config/config.json
{
...
...
    "hooks": {
    "extra_custom_fields": ["eshop_extra_custom_fields"]
    }
}

#/themes/e_shop/config/Gemfile
gem 'country_select'

#/themes/e_shop/main_helper.rb
module Themes::EShop::MainHelper
  def eshop_extra_custom_fields(args)
    args[:fields][:my_country_select] = {
        key: 'my_country_select',
        label: 'Country Selector',
        render: theme_view('custom_field/country_select.html.erb'),
        options: {
            required: true,
            multiple: true,
        }
    }
  end
end