Add custom activity marker

Hey guys, I’m new to the BPMN world and I have a task where I have to add an activity marker.
What I actually need is to add a plus icon here:
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Any help on where to start will be really appreciated

So i know that i need to create a custom popupmenu or better extend the existing one, use as example:

Then I uncommented the getHeaderEntries to generate the example button, but nothing works.
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direcotry structure:
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this is the content of index.js that returns the custom popupmenu:
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This is then this is how called everything in the main index.js

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Hey,

Could you please share a sandbox of what you’ve already done: https://codesandbox.io/s/bpmn-js-sandbox-3zkw0

We can then help you with that : )