The documentation for InnerBlock has a prop renderAppender which can be used to add a custom button. In the example:

// Fully custom
<InnerBlocks
    renderAppender={ () => (
        <button className="bespoke-appender" type="button">Some Special Appender</button>
    ) }
/>

the custom button does nothing on click. How can open the Block Picker Menu on click of the custom button?