How to use the com.hybris.cockpitng.editor.defaultmap Editor in cockpitng

Hi, we try to use the maptype in our items.xml, because we have many fields which have value per lets call it "CustomMapKeyType". So we declare in the items.xml:

             <maptype code="custommappable:java.lang.String"
                 argumenttype="CustomMapKeyType"
                 returntype="java.lang.String"
                 autocreate="true"
                 generate="false"/>

     <itemtype code="CustomMapKeyType"
                  jaloclass="de...jalo.CustomMapKeyType"
                  autocreate="true"
                  generate="true">
         <attributes>
             <attribute qualifier="code" type="java.lang.String" >
                 <persistence type="property"/>
             </attribute>
             
             <attribute qualifier="name" type="java.lang.String">                    
                 <modifiers initial="true" optional="false" />
                 <persistence type="property"/>
             </attribute>
             ...

         </attributes>        
         <indexes>
             <index name="CustomMapKeyTypeCodeIx" unique="true">
                 <key attribute="code" />
             </index>
             ...
         </indexes>
     </itemtype>

and then for some other type's property like this:

 <attribute qualifier="custommappableTitle" type="custommappable:java.lang.String">

In the -backoffice-config.xml we have for the editor area:

 <ea:attribute qualifier="custommappableTitle" />

When we show our custommappableTitle field in the editor it is picking the "com.hybris.cockpitng.editor.defaultmap" editor, but if I click on the "+" sign to add values it is showing empty value for "Key". In the hmc I am able to fill this attribute and the values added are shown in the cockpitng with the CustomMapKeyType(PK) format.

Do you have any hints how to populate the "Key" field for the "com.hybris.cockpitng.editor.defaultmap" editor when adding value?