hypothetically - if i have an extension and i want it to show as a submenu by using menus.ui, how can i call it from there? 
thanks. 

View>Command Palette

Search for extension action.

For example 

org.inkscape.color.randomize

Appears as

doc.org.inkscape.color.randomize

Copy the .ui file to your user profile ui folder.

Then just add an extra entry to the end.

-----

For example for toolbar-zoom.ui

add the following entry after the last child

make sure <object class="GtkImage" id="image13">

Is edited to make the id unique, otherwise Inkscape will not load.

choose an icon file from inkscape/icons/Dash/symbolic/actions

for example 

<property name="icon-name">bitmap-trace-symbolic</property>

( do not include the .svg )

 <child>
      <object class="GtkButton">
        <property name="visible">True</property>
        <property name="tooltip-text" translatable="yes">Zoom Next</property>
        <property name="action-name">doc.org.inkscape.color.randomize</property>
        <property name="relief">none</property>
        <child>
          <object class="GtkImage" id="image13">
            <property name="visible">True</property>
            <property name="icon-name">bitmap-trace-symbolic</property>
          </object>
        </child>
      </object>
      <packing>
        <property name="position">15</property>
      </packing>
    </child>
    
    
It's better to launch Inkscape from the terminal or command line first.

Any errors will appear on the command line if Inkscape does not load.