Create submenu within menu flexdashboard

Hello,
I am able to create a submenu in flexdashboard using e.g.:

Healthy child programme {data-navmenu="Programmes"}
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Health protection programme {data-navmenu="Programmes"}
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NHS Health check programme {data-navmenu="Programmes"}
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I want to create a submenu within each programme so within the Health protection programme I want 'screening' and 'vaccinations'. I have found CSS code to create a submenu but how do I designate a submenu if the above designates a menu? I am not using Shiny

Thanks