Add those two lines to your HTML header:
<link rel="stylesheet" href="mastodonFollow.css">
<script src="mastodonFollow.js"></script>Add a button which opens the Mastodon remote follow page:
<button class="mastodon-follow" data-followtype="remote" data-account="@[email protected]"></button>Add a button which asks for the home instance of the user and then allows them to directly follow you:
<button class="mastodon-follow" data-followtype="direct" data-account="@[email protected]"></button>The account has to be in the format "@username@instance"
Simple demo: https://autinerd.github.io/simple-mastodon-follow-button/