Add <bpmn:property> to <bpmn:process>

I’m trying to programatically add a <bpmn:property> element process to the <bpmn:process> element, but I’m struggling to find the best way to do it.

It feels like it should be a simple thing to do, so could anyone give me any pointers as how best to approach this please?

What did you already do? Can you share something in a Sandbox?

Here you go: