app.html.erb

<div>
  <% if flash[:notice].present? %>
    <script>
      $(document).on("turbolinks:load", function() {
        $(".page-heading").after("<div class='flash flash-notice'><%= flash[:notice] %></div>");
      });
    </script>
    <% flash.clear %>
  <% end %>
  <%= yield %>
</div>

order_products_controller.rb

class OrderProductsController < ApplicationController
   def update
     @order_product = OrderProduct.find(params[:order_product][:product_id])
     @order_product.dropbox_link = params[:order_product][:dropbox_link]
     @order_product.save
     respond_to do |format|
       if @order_product.save
         format.js
       else
         format.html do
           flash[:notice] = "Please include https:// to the beginning of your dropbox link."
           redirect_to "/dashboard"
         end
       end
     end
   end

   def edit
     @product = OrderProduct.find(params[:id])
     @designers = Designer.all
     @printers = Printer.all
   end

   private

   def products_params
     params.require(:order_product).permit(:product_id, :dropbox_link)
   end
end