when I scroll the page your submenu is pinned on the top.
I want to use this function.
how can I do it?
To recreate this display, you will have to create a slider with a Revolution Slider, with links that will imitate a menu a the top right corner, and choose the header Below slider style in Betheme -> Theme options -> Header & Subheader -> Header.
If you do not know how to use the Revolution Slider plugin, please, check their official manual.