Properties panel extension

I am trying to add some custom elements under the bpmn:ExtensionElements. I’m using properties panel but I don’t know how to add the child elements under the specific element.

e.g. I want to add
bpmn:ExtensionElements
<custom:abc id=“random_id” visible=“boolean”>

MONDY
TUESDAY

</custom:abc>
</bpmn:ExtensionElements>

How I can achieve the above goal?

Regads,
Ali