Used in https://stackblitz.com/edit/monolithic-reactive-form

app.component.ts

import { Component, OnInit } from '@angular/core';
import {FormGroup, FormBuilder} from '@angular/forms';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.scss' ]
})
export class AppComponent implements OnInit {
  checkoutForm: FormGroup;

  constructor(private fb: FormBuilder) {}

  ngOnInit() {
    this.checkoutForm = this.fb.group({
      fullName: null
    })
  }

  /**
   * After a form is initialized, we link it to our main form
   */
  formInitialized(name: string, form: FormGroup) {
    this.checkoutForm.setControl(name, form);
  }

}