x-menu-bar-open-internal is an interactive built-in function in `C source code'.

(x-menu-bar-open-internal &optional FRAME)

Start key navigation of the menu bar in FRAME.
This initially opens the first menu bar item and you can then navigate with the
arrow keys, select a menu entry with the return key or cancel with the
escape key. If FRAME has no menu bar this function does nothing.

If FRAME is nil or not given, use the selected frame.