Getting error in OnAction in Ribbon powerpoint

I'm trying to define a dynamic combobox in PowerPoint's Ribbon using Office Ribbon X Editor. When I enter the onAction line I get an error:

Ln 14, Col 25: The 'onAction' attribute is not declared.

Code:

<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui">
    <ribbon startFromScratch="false">
        <tabs>
            <tab id="customTab" label="Custom Tab">
                <group id="customGroup" label="Custom Group">
                    <button id="customButton" label="Custom Button" imageMso="HappyFace" size="large" onAction="Callback" />
                
                     <comboBox 
                        id="comboBox"
                        label="Size" 
                        sizeString="AAAA"
                        getItemCount="GetCBItemCount"
                        getItemID="GetCBItemID"
                        onAction="ddClicked"
                        >
                     </comboBox>
                    </group>
            </tab>
        </tabs>
    </ribbon>
</customUI>

Any help appreciated

There is no onAction on combobox. Try onChange

<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui">
    <ribbon startFromScratch="false">
        <tabs>
            <tab id="customTab" label="Custom Tab">
                <group id="customGroup" label="Custom Group">
                    <button id="customButton" label="Custom Button" imageMso="HappyFace" size="large" onAction="Callback" />
                
                     <comboBox 
                        id="comboBox"
                        label="Size" 
                        sizeString="AAAA"
                        getItemCount="GetCBItemCount"
                        getItemID="GetCBItemID"
                        onChange="ddClicked"
                        >
                     </comboBox>
                </group>
            </tab>
        </tabs>
    </ribbon>
</customUI>