Example adding custom copy button code block pro

copyscript.js

const blocks = Array.from(
    document.querySelectorAll('.wp-block-kevinbatdorf-code-block-pro'),
);
const copyText = 'Click anywhere to copy';
const copiedText = 'Copied!';
blocks &&
    blocks.forEach((block) => {
        if (!block.querySelector('span[data-code]')) return
        block.insertAdjacentHTML(
            'beforeend',
            `<span class="my-copy-button" style="position:absolute;top:0;right:0;color:white;padding:0 0.5rem;background:#2e2e2e;z-index:9999;height:36px;font-size:0.9em;display:flex;align-items:center;">${copyText}</span>`,
        );
        block.addEventListener('click', () => {
            block.querySelector('span[data-code]').click();
            const copyButton = block.querySelector('.my-copy-button');
            copyButton.innerText = copiedText;
            setTimeout(() => (copyButton.innerText = copyText), 1500);
        });
    });

Requires the copy button to be there. Will just overlay some text on top of it.