Easiest way to do this would be to have an about or contact page, and include the link.
<a rel="me" href="<mastodon profile link>Find me on Mastodon</a>
If you don’t want to show the link on your page. For example, you want to be verified on mastodon, but don’t want visitors to see the link on your Write.as page, use this:
<a class="masto-link" rel="me" href="[mastodon profile link]">Find me on Mastodon</a>
Then add the following CSS in your customiser:
a.masto-link { display:none; }