Is this possible for me to do this? And if so, how?
I have html coding experience, just don’t know / can’t find where to look to add this link in which can be hard coded to an additional section on the website.
In addition, I only want to display this to certain customers who have the tag “buildmanual” on their account.
You will have to use {% if customer.tags contains ‘buildmanual’ %} in your theme files most probably in your header.liquid file and then in that logic, you will have to update the link for it. It’s quite a complicated task so I would highly recommend you to hire a Shopify developer for that. If you want me to help you out then feel free to connect with me and I would be more than happy to give you a helping hand!
With the Shopify new customer accounts system, it is NOT possible to edit this directly through the theme code as some others have suggested. The new customer account pages are hosted directly by Shopify and are not part of your theme.
Currently, it’s not possible to edit the navigation items available on the new customer accounts pages, however this is something that’s available in developer preview, so I expect Shopify will be releasing this feature publicly soon, Once this feature is available, you will be able to access and edit your customer accounts menu through the ‘Content’ section of your Shopify admin: