Assuming that you have properly set your Base URL in Admin > Settings > Site information, you can add a hyperlink just by including a slash and the target slug. For example, this would link to the default "About" page, which has a slug of /about in AtoM:
- This is my [hyperlink](/about).
You can see an example of a Markdown link constructed this way in the default Privacy notification banner message we include in AtoM - here's a screenshot of it from our demo site:
You can also achieve this using HTML:
- This is my <a href="/about">hyperlink</a>.
For tips on formatting with Markdown, see:
For some suggestions on using HTML to style static pages instead (something that is ONLY supported on static pages, and not in records), see:
If you use HTML, be sure to review the documentation link above, as there are some quirks to using HTML on AtoM static pages - for example, to ensure that certain multi-tag elements (like tables) render properly, you need to add them all together in a single line without spaces (making it less human-friendly to read and write, unfortunately) - otherwise, some of the other page helpers will interfere with the rendering.
You will have the best results if you don't mix HTML and Markdown in a static page - technically they CAN be mixed in a page but not in a single section, but to avoid possible complications, it is generally recommended to use Markdown whenever possible.
Cheers,