Not sure why the responses aren't visible to you but they don't give an explicit answer anyway. The key response contains this document;
support.toastmastersclubs.org/doc/item/agenda-templates
See the section titled "Save an existing agenda as a template". Note; this is not done in the Meeting Agenda Settings console.
This must be done by opening a currently dated agenda in edit mode (the one you wish to duplicate), click on the 'Agenda Setup' tab then on the button "Save this agenda as an Agenda Template." and give it the desired name.
Once that is complete you can go to the 'Meeting Agenda Settings' console and you'll find the new agenda in the drop-down list awaiting your adjustments.