addLink() {
 //when the plus button is clicked

   const placeholdervalue = this.addForm.get("placeholdervalue").value;
   this.items = this.addForm.get("items") as FormArray;
    this.items.push(this.createItem(placeholdervalue));
    console.log(this.addForm.get("items"));
  }

  createItem(placeholdervalue: string): FormGroup {
    let a = { [placeholdervalue]: "" };
    return this.formBuilder.group(a);
  }
  ngOnInit() {
    this.addForm = this.formBuilder.group({
      items: this.formBuilder.array([]),
      placeholdervalue: ""
    });
  }
}

<div class="row">
            <div
              class="col-md-3"
              formArrayName="items"
              *ngFor="
                let item of addForm.get('items').controls;
                let i = index
              "
            >
              <div [formGroupName]="i">
                <mat-form-field class="example-full-width">
                  <input
                    matInput
                    formControlName="" // i want to retrieve it from item
                    placeholder=""
                  />
                </mat-form-field>
              </div>
            </div>
          </div>