You can see in top Custom tab, on click your previews custom link list open.
You can see Add Custom URL button next to search option, on click popup window open.
In popup you need to add Title, it’s only for backend listing purpose.
After add title, you need to add your URL which should start with your shopify domain name. Ex: ‘https://mystore.myshopify.com/contact’ is your page url then you need to enter only ‘contact’ in input. System automatic complete rest url and generate your short URL.
After generating list you can see different copy URL and view button in the same row for the copy short URL or view analytics result of this link..
If you want to see original URL with short URL, you need to click View button which is at the last, it will open detail page of this link.