Form event for onFormPopulated

Dears, I want the open form every time  then display the field value. And the field value is not disapear when I lock or unlock . The field data type is Federated. Which one do I use? And which form event can I used ? (ex: onFormPopulated ? onLoad?) A：handleItemChange("_name", "AAA"); B：document.thisItem.setProperty("_name", "AAA");