how to send CTRL-ALT-Fn to virtual box guest Linux?

Host + F1, default Host key is Right Ctrl.

Long answer: The quickest path to the answer is actually VirtualBox's excellent manual (yes, some people still write those things these days). You can open the manual with the menu Help -> Contents... and type "ctrl+alt+f" in the search field: this will position you right to the chapter "Typing special characters", with the answer you're looking for. Otherwise, you can also read it online: https://www.virtualbox.org/manual/ch01.html#specialcharacters

Short answer: We tend to rely too much on stackexchange, while we just need to RTFM to find a quality answer. (me included :)