Adding bpmn:ExtensionElement to user task

Hello everyone,

I am using properties-panel-extension example, creating a browserify bundle of properties panel + modular.
My problem is that i am not able to add an extension element under bpmn:usertask.
I am following the process in the example, and i figured out that it just adds an attribute to the element that i choose. But the thing is i want to create an extension element and inside i want to add form data with custom inputs.
it is similar to the one that is already found in camunda but i have custom fields that i want to add.

If anyone could help, I would highly appreciate it.