To add an external link (such as ambassador portal) to navigation, go to Online Store > Navigation.
Then you can select your Footer or Main menu.
The main menu appears at the top of your store and will get more views. The footer appears at the bottom of your Shopify website.
I recommend the Main menu, but it depends on the layout and styling of your store – so you’ll have to decide what makes sense for your business.
Once you’ve selected a menu, click Add menu item and type in a name for your link. Since I'm adding an ambassador portal link - I'll use "Become An Ambassador".
In the link field, paste in your link.
Make sure to click the link after you’ve pasted in it or it won't work.
See the red box in these screenshots:
Then click the green Add button.
Afterwards, you should end up with something like this:
Congrats! You’ve added an external link to your Shopify store navigation!
If you’re getting an error, I recommend deleting your link and refreshing your browser.
Then try re-adding the link.
Make sure you click the link after you paste it in. See this screenshot, click where the redbox is: