Hi Stefan
You should use:
   ((TIETextLayer*)ImageEnView1->CurrentLayer)->Text = "Test";
Or:
void __fastcall TForm1::Button1Click(TObject *Sender)
{
  ImageEnView1->LayersAdd(ielkText);
  TIETextLayer *txtlayer;
  txtlayer = dynamic_cast<TIETextLayer*>(ImageEnView1->CurrentLayer);
  if (txtlayer != NULL)
    txtlayer->Text = "This is a test text message";
  ImageEnView1->Update();
}
https://docwiki.embarcadero.com/RADStudio/Sydney/en/Dynamic_cast_(C%2B%2B_Type_Cast_Operator)
https://docwiki.embarcadero.com/RADStudio/Sydney/en/Static_cast(typecast_Operator)
Nigel 
Xequte Software
www.imageen.com