Welcome
There are two solutions without modifying the code.
1. If you create the portfolios page via BM content builder, set source as “Media Library” instead of “portfolio”. You can also set a custom URL(URL of external website) in the WordPress “Media Library”.(check video guide at 04:40)

2. You can install a redirect plugin, redirect the single portfolio post URL to the URL of external website.
Regards!
SeaTheme – FAQs – WordPress Customization – Facebook