Add those two lines to your HTML header:
<link rel="stylesheet" href="mastodonShare.css">
<script src="mastodonShare.js"></script>Add a button which shares the current page:
<button class="mastodon-share"></button>Add a button which shares another page:
<button class="mastodon-share" data-title="Join Mastodon!" data-href="https://joinmastodon.org"></button>Simple demo: https://autinerd.github.io/simple-mastodon-share-button/