active_admin.rb

# config/initializers/active_admin.rb

require 'active_admin_custom_filter'

ActiveAdmin.setup do |config|
  # ...
end

active_admin_custom_filter.rb

# lib/active_admin_custom_filter.rb

module ActiveAdmin

  class FilterFormBuilder
    include ::ActionView::Helpers::OutputSafetyHelper

    def filter_custom_input( method, options = {} )
      field_name = method

      safe_join(
        [
          label( field_name, I18n.t( 'active_admin.search_field', :field => options[:label] ) ),
          text_field( field_name )
        ],
        "\n"
      )
    end

  end

end

resource.rb

# app/admin/resource.rb

ActiveAdmin.register Resource do
  filter :first_name_or_last_name_contains, :as => :custom, :label => 'Name'
end