We use the WYSIWYG text formatter which interprets HTML using the definite rules. If you want to add
URL shortening service, you should figure out how the WYSIWYG functionality and
URL shortening service work and create your custom plugin.
As far as I know it's quite complicated to integrate the
URL shortening service which will creates/store the shorten URLs and required a lot of hard coding, but not adding a small java-script. So, the only way to create this feature is developing a new custom plugin.
I suggest that you download our new Plugin Skeleton plugin to see how to use this or that function. Here is URL to the Store item:
http://www.oxwall.org/store/item/695
Also, please see the crash course for more information on how to create your own plugin here:
http://docs.oxwall.org/dev:crash-course
If you are not familiar with the plugin development, it would be better for you to find a developer who can help you with this plugin development. I suggest visiting our Oxwall Market