I want to add domain.com/admin to every domain's folder. How do I do this?
For you to add the /admin folder to every domain to access Helm you would have to do this manually on each domain.
Helm does not support this because Helm is multi-server based and not installed on each individual server.
What you can do is add a virtual directory in IIS to each domain called /admin, then in the IIS properties for this virtual directory set it to redirect to another URL and enter the URL of your Helm control panel.