[rails_admin] How to create a costum action with form? GET and POST

next_step.html.erb

<!-- <root>/app/views/rails_admin/main/next_step.html.erb -->
<div class="container-fluid">
	<div class="row">
		<div class="col-md-6 col-md-offset-3">
			<%= form_tag(next_step_path, {method: :post}) do %>
				<div class="form-group">
					<label for="step">Próximo Passo:</label>
					<%= select_tag "step", options_from_collection_for_select(@kind_steps, "id", "name"), prompt: "Selecione o Passo", class: "form-control", onchange: "this.form.submit();" %>
				</div> 
			<% end %>
		</div>		
	</div>	

	<div class="row">
		<div class="col-md-8 col-md-offset-2">
			<table class="table">
    			<thead>
      				<tr>
        				<th>Passo</th>
        				<th>Responsável</th>
        				<th>Em</th>
        			</tr>
    			</thead>
    			<tbody>
    				<% @object.steps.order(created_at: :desc).each do |step|%>
      				<tr>
        				<td><%= step.kind_step.name %></td>
        				<td><%= step.user.name %></td>
        				<td><%= l step.created_at %></td>
        			</tr>
        			<% end %>
        		</tbody>
        	</table>
		</div>		
	</div>	
</div>

next_step.rb

# <root>/lib/rails_admin/next_step.rb
class RailsAdmin::Config::Actions::NextStep < RailsAdmin::Config::Actions::Base
    RailsAdmin::Config::Actions.register(self)

    register_instance_option :http_methods do
          [:get, :post] # NEW / CREATE
    end

    register_instance_option :member do
        true
    end

    register_instance_option :breadcrumb_parent do
        [:show, bindings[:abstract_model], bindings[:object]]
    end

    register_instance_option :controller do
        proc do
            if request.get?
                # Rendering the page
                @kind_steps = @object.kind.kind_steps
                render @action.template_name, status: 200
            elsif request.post?
                @step = Step.new(demand: @object, kind_step_id: params[:step].to_i, user: current_user)
                if @step.save!
                    @auditing_adapter && @auditing_adapter.create_object(@object, @abstract_model, _current_user)
                    respond_to do |format|
                        format.html { redirect_to_on_success }
                        format.js   { render json: {id: @object.id.to_s, label: @model_config.with(object: @object).object_label} }
                    end
                else
                    handle_save_error
                end
            end
        end
    end

    register_instance_option :route_fragment do
        'next-step'
    end

    register_instance_option :link_icon do
        'icon-arrow-right'
    end

    register_instance_option :visible? do
        subject = bindings[:object]
        subject.is_a? Demand
    end
end

rails_admin.rb

# <root>/config/initializers/rails_admin.rb
RailsAdmin.config do |config|
  require Rails.root.join('lib', 'rails_admin', 'next_step.rb')

  ### Popular gems integration

  ## == Devise ==
  config.authenticate_with do
    warden.authenticate! scope: :user
  end
  config.current_user_method(&:current_user)

  ## == CancanCan ==
  # config.authorize_with :cancancan

  ## == Pundit ==
  # config.authorize_with :pundit

  ## == PaperTrail ==
  # config.audit_with :paper_trail, 'User', 'PaperTrail::Version' # PaperTrail >= 3.0.0

  ### More at https://github.com/sferik/rails_admin/wiki/Base-configuration

  ## == Gravatar integration ==
  ## To disable Gravatar integration in Navigation Bar set to false
  # config.show_gravatar = true

  config.actions do
    dashboard                     # mandatory
    index                         # mandatory
    new
    export
    bulk_delete
    show
    edit
    delete
    show_in_app
    next_step

    ## With an audit adapter, you can add:
    # history_index
    # history_show
  end
end