application.html.erb

<!-- layouts/application.html.erb -->
<div data-controller="modal">
  <%= link_to "Get Started", "#", data-action="modal#show" %>

  <div data-target="modal.dialog" style="display: none;">
    <% form_for authentications_path do |f| %>
      <!-- Form markup -->
    <% end %>
  </div>
</div>

modal_controller.js

export default class extends Controller {
  static targets = ["dialog"]

  show() {
    this.dialogTarget.style.display = "block";
  }
}