title: How to add value to a form group using add control
body: <p>How do we add controls to a formgroup ? I tried the ff below but it does not add. Any idea? thanks.</p>
  <pre><code>modelForm: FormGroup;
  
  //this happens on ngOnit
  this.modelForm = this._createModelForm();
  
  private _createModelForm(): FormGroup {
      return this.formBuilder.group({
        id: [this.model.id || 0],
        summary: this.model.summary,
        dealType: this.model.dealType,
        dealTypeValues: this.model.dealTypeValues,
      });
  
    }
  </code></pre>
  <p>For example I want to add</p>
  <p><code>this.modelForm.addControl('dealType',new FormControl('Partner Location Submission'));</code></p>
  <p>but does not seem to work, any idea guys? thank you.</p>