How to hide header before scrolling on landing page


For my landing page, I want to hide the header with nav menu until we start scrolling down. Can someone please help me with this? This is my website:

I saw on an old post Start with no menu, show only after scrolling, I tried to add css like what he suggested, but it did not work.

I am very new to this, so I am not sure if I missed out on any step.

Please help!

Thank you very much in advance.

Best Regards,