ElementInternals.form

let form = ElementInternals.form;

<form id="myForm"><custom-checkbox id="join-checkbox"></custom-checkbox>
  <custom-label for="custom-checkbox">Join newsletter</custom-label></form>

class CustomCheckbox extends HTMLElement {
  static formAssociated = true;

  constructor() {
    super();
    this.internals_ = this.attachInternals();
  }

  /* ... */

  window.customElements.define("custom-checkbox", CustomCheckbox);
})();

let element = document.getElementById("custom-checkbox");
console.log(element.internals_.form.length);