Add attributes to existing product type

Hi

I am a bit confused on how to accomplish this.

I want to have a new attribute on the product cockpit when editing a existin product type. And I also want to have this new attribute on the rest response.

I had created a custom extension for OCC and inside myextension-items.xml I added the following:

 <itemtype code="Product" generate="false" autocreate="false">
     <attributes>
         <attribute qualifier="url_key" type="localized:java.lang.String">
             <modifiers read="true" write="true" search="true" optional="true" private="true"/>
             <persistence type="property"/>
         </attribute>
     </attributes>
 </itemtype>

Then compiled and updated. Do I need to do anything else? I don't see the field anywhere.

Thanks for the help