add control to formgroup code example

Example 1: formgroup addcontrol

let form: FormGroup = new FormGroup({});
form.addControl(field_name, new FormControl('', Validators.required));

Example 2: formgroup angular

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      const form = new FormGroup({
  first: new FormControl('Nancy', Validators.minLength(2)),
  last: new FormControl('Drew'),
});

console.log(form.value);   // {first: 'Nancy', last; 'Drew'}
console.log(form.status);  // 'VALID'