In addition to linking to your published pages, you can include arbitrary URLs such as external websites, campaign landing pages, or documents by leveraging a Custom Links Content Model.
Move to the navigation section and select your menu.
In the left panel, choose the Custom Links model.
Tick the checkbox next to the entries you want in your menu.
Click Add to Menu; the items appear in the right pane.
Drag and drop to position them alongside page links.
Click Save to update the menu.
With Custom Links integrated into your navigation, you can easily blend internal pages and external resources giving users a unified menu experience across all your site’s destinations.