This question could be solved with a combination of these two answers: https://stackoverflow.com/a/43493648/6294072 and https://stackoverflow.com/a/47670892/6294072

So first of all, you would need a custom validator for checking the passwords, that could look like this:

checkPasswords(group: FormGroup) { // here we have the 'passwords' group
  const password = group.get('password').value;
  const confirmPassword = group.get('confirmPassword').value;

  return password === confirmPassword ? null : { notSame: true }     
}