Accessing addon domain in browser:

As cPanel treats addon domain as a subdomain for main account so it is accessed in web browser as a subdomain but if you  want to access the addon domain in web browser as:

http://addondomain and not as as sudomain like :http://maindomain/addon domain.

In that case we needs to create a .htaccess and define below rules:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST} main-domain\.com$
RewriteRule ^(.*)$ "http\:\/\/www.another-domain\.com\/" [R=301,L]

If you want to redirect your website using .htaccess you can check here.