VBA XML Ribbon Callback to xlam Class Instance?

I'm building an xlam add-in and whilst I'm able to callback to standard module subs, I'd like to be able to callback to an instance of a class module

I'd tried something like the below, but it didn't work - any idea if it's possible?

Code:

---

<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui">
  <ribbon>
    <tabs>
      <tab id="TestTab" label="TestTab">
        <group id="TestGroup" label="Test Group">
          <button id="TestButton" label="Test" onAction="'MyAddIn.xlam'!ThisWorkbook.TestClassInstance.TestSub"/>
        </group>
      </tab>
    </tabs>
  </ribbon>
</customUI>

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Quote:

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to an instance of a class module

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i would believe you should have the class instance held in a variable in your public module

doesn't seem to matter where I put the class instance, I've tried both in ThisWorkbook and in a module - it says i can't find the macro.

Also tried referring to the class instance without the location of where it's declared, but that didn't work either:

Code:

---

'MyAddIn.xlam'!TestClassInstance.TestSub

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