_form.html.erb

<%= form_for(board, html: {class: "space-y-4"}) do |form| %>
  <div>
    <%= form.label :title, class: "block text-sm font-medium text-gray-700" %>
    <%= form.text_field :title %>
  </div>

  <div>
    <%= form.fields_for :embeds do |embed_form| %>
      <%= embed_form.label :input, class: "sr-only" %>
      <%= embed_form.text_field :input, placeholder: "Enter an embeddable link", data: {reflex_permanent: true} %>
    <% end %>

    <div class="relative">
      <div class="absolute inset-0 flex items-center" aria-hidden="true">
        <div class="w-full border-t border-gray-300"></div>
      </div>
      <div class="relative flex justify-center">
        <%= button_tag
            data: {reflex: "click->NestedForm#add", resource: "Board", nested_resource: "Embed"} do %>
          <svg class="-ml-1.5 mr-1 h-5 w-5 text-gray-400" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20" fill="currentColor" aria-hidden="true">
            <path fill-rule="evenodd" d="M10 5a1 1 0 011 1v3h3a1 1 0 110 2h-3v3a1 1 0 11-2 0v-3H6a1 1 0 110-2h3V6a1 1 0 011-1z" clip-rule="evenodd" />
          </svg>
          <span>Add Embed</span>
        <% end %>
      </div>
    </div>
  </div>

  <%= form.submit" %>
<% end %>

boards_controller.rb

class BoardsController < ApplicationController
  #...
  def new
    @board = session.fetch(:forms, {}).fetch("Board", Board.new)
  end
  
  def create
    @board = Board.new(board_params)

    respond_to do |format|
      if @board.save
        session[:forms].delete "Board"
        format.html { redirect_to @board, notice: 'Board was successfully created.' }
      else
        format.html { render :new }
      end
    end
  end
  
  # ...
end

nested_form_reflex.rb

class NestedFormReflex < ApplicationReflex
  def add
    session[:forms] ||= {}
    session[:forms]["Board"] ||= Board.new

    session[:forms]["Board"].embeds.build
  end
end