leilukin-site/src/_includes/main/site-btn.njk

<div class="site-btn__container">
    <div>
        <a href="https://leilukin.neocities.org/" target="_blank"><img src="/assets/leilukin/Leilukins-Hub-button.png" alt="Button of Leilukin's Hub" title="Leilukin's Hub"></a>
    </div>
    <textarea class="site-btn__textarea textarea-for-copy" rows="4"><a href="https://leilukin.neocities.org/" target="_blank"><img src="https://leilukin.neocities.org/assets/leilukin/Leilukins-Hub-button.png" alt="Button of Leilukin's Hub" title="Leilukin's Hub"></a></textarea>
    <button class="copy-txt-btn">Copy HTML</button>
</div>

<style>
    .site-btn__container {
        display: flex;
        gap: 1rem;
        align-items: center;
        margin: 0.5rem 0;
    }

    .site-btn__textarea {
        width: 10rem;
        height: 4rem;
        font-size: 0.75rem;
        background-color:black;
        color: var(--clr-body-txt);
    }

    .copy-txt-btn {
        border: none;
        background: var(--clr-link-btn-bg);
        color: var(--clr-body-txt);
        padding: 0.3rem 0.8rem;
        border-radius: 0.5rem;
        font-size: 0.9rem;
        font-weight: bold;
    }

    .copy-txt-btn:hover {
        background: var(--clr-link-hover);
    }
</style>

<script defer>
    document.querySelector(".copy-txt-btn").onclick = () => {
        document.querySelector(".textarea-for-copy").select();
        document.execCommand('copy');
    }
</script>