test.ts

@HostListener('document:txDocumentEditorLoaded', ['$event'])
onTxDocumentEditorLoaded() {
  // wait until TXTextControl has been loaded
  TXTextControl.addEventListener("textControlLoaded",  () => {
    TXTextControl.loadJsonData(this.mailMergeData.json);
  });
}

ngOnInit() {
  this.getJsonData();
}

getJsonData() {
  this.http.get<MailMergeData>('/mailmerge/getjsondata').subscribe(
    (result) => {
      this.mailMergeData = result;
    }
  );
}