Pass GUID to HTML web resource for Navigation Link

Hi -

I want to be able to open a URL via a navigation link in the contact entity. I need to pass the GUID of the contact record to the URL, so it becomes ".com?contactid=GUID".

The article here (http://www.crmanswers.net/2013/03/dynamic-navigation-links-using.html) is really useful in pointing me in the right direction, but it refers to passing the name as a parameter. How can I do it with the contact ID guid instead?

On CRM 2015 on-prem.

Thanks.