i want to open the boot menu but i dont know how
On boot, either: 1. hold shift key 2. repeatedly tap esc key.
if I am not taking your question wrong then this is something device/hardware specific. some devices do it with esc, some with F2, some F8, some F12. mostly it's esc. but you can google "how to enter boot menu [manufacturer name]"
That would be correct for BIOS. OP is asking about boot menu.