application.html.erb

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

modal_controller.js

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

  show(event) {
    fetch(event.currentTarget.href).then(data => {
      this.dialogTarget.innerHTML = data;
    })
  }
}