gistfile1.html

<form #registerForm="ngForm">
  <div class="form-group">
    <label for="username">Username</label>
    <input [(ngModel)]="formData.username"
          #username="ngModel"
           name="username"
           type="text"
           class="form-control"
           id="username"
           required>
     <div *ngIf="username.invalid && (username.dirty || username.touched)"
         class="alert alert-danger">

      <div *ngIf="username.errors.required">
        Username is required.
      </div>
    </div>
  </div>

  <div class="form-group">
    <label for="email">Email</label>
    <input [(ngModel)]="formData.email"
           name="email"
           type="email"
           class="form-control"
           id="email"
           required>
  </div>

  <div class="form-group">
    <label for="password">Password</label>
    <input [(ngModel)]="formData.password"
           name="password"
           type="password"
           class="form-control"
           id="password"
           required>
  </div>

  <div class="form-group">
    <label for="passwordConfirmation">Confirm Password</label>
    <input [(ngModel)]="formData.passwordConfirmation"
           name="passwordConfirmation"
           type="password"
           class="form-control"
           id="passwordConfirmation"
           required>
  </div>
  <button [disabled]="!registerForm.form.valid" (click)="register()" type="submit" class="btn btn-bwm">Submit</button>
</form>