如何使一个在angular中已经存在默认值的输入字段成为必填字段？

<input type="text" [(ngModel)]="myValue" required>

export class MyComponent {
  myValue: string;
  isFilled: boolean = false;
}

<form (ngSubmit)="submitForm()">
  <input type="text" [(ngModel)]="myValue" required>
  <button type="submit">Submit</button>
</form>

export class MyComponent {
  myValue: string;
  isFilled: boolean = false;

  submitForm() {
    if (this.myValue) {
      this.isFilled = true;
      // 执行其他操作
    }
  }
}

<form (ngSubmit)="submitForm()">
  <input type="text" [(ngModel)]="myValue" required>
  <div *ngIf="!isFilled">Please fill in this field.</div>
  <button type="submit">Submit</button>
</form>