get the ‘moddle’ module from the bpmnDiagram and create the listener like this:
var moddle = bpmnDiagram.get('moddle');
var extensionElements = moddle.create('bpmn:ExtensionElements');
var executionListener = moddle.create('camunda:ExecutionListener');
executionListener.class = 'myCustomDelegateClassName';
executionListener.event = 'start';
extensionElements.values.push(executionListener);
now update the properties using the modeling.
var modeling = bpmnDiagram.get('modeling');
modeling.updateProperties(element, {extensionElements: extensionElements});