I wanted to add a custom string property on the URL/LinkItemCollection property similar to "Remaining Url".
See this blog post from Ynze for extending the URL editor, it's for CMS 11 but I don't see why it couldn't work for CMS 12
Thanks for the reply, Ron!.
It is working fine in CMS 12 but it adds the field as a custom link field and I want to use this custom field as a standard text field but the code group it in Links so I can either choose a link or the Custom field
Did you add the dojo class for the custom field?
See the full example referenced in Ynze's blog