Adding None HTTP based URL link to navigation

Greetings Ghost community.

I would like to add a none http based href into the navigation (primary or otherwise) unfortunately the URL validation is stopping me from doings so.

For example I would like to add a “tel:1800123456” in the primary navigation.

I would like to avoid editing the theme directly as I am using Casper for the moment and I want to continue to get updates until I can afford the time/money to have my own theme maintained.

Any help or general suggestions would be greatly appreciated.

Cheers and Beers