There might be a situation in which you want to add custom links to the sidebar menu of your user account pages. This can be done fairly easily with JavaScript.
- Log into your Store and navigate to "System → Modules → Custom Tags"
 - Click on New button on top right
 

- Set the Position to "Footer - directly after </body> and before </html>"
 - Set Page Type to "User Page"
 - Add your code and click on Save
 

You can use this code as a template:
<script> 
$(document).ready(function(){ $('.my-account-sidebar ul').append('<li class="custom_link">
<a href="{YOUR URL HERE}">{MENU LINK TITLE}
</a></li>'); }); 
</script>
- Update the {YOUR URL HERE} with the URL to the page you want to link to.
 - Change the {MENU LINK TITLE} to the text you want for this menu item.
 
Press the "Update" button in the top-right of the page to save your changes once done.

