I have an HDD with installed Ubuntu 18.04 and use it as an external OS to run from my macbook and any windows os.
I can use “alt” button from the starting my macbook to use Ubuntu but I don’t know how to open from windows 8.1.
How can I achieve to open HDD installed Ubuntu from windows 8.1 ?
If I understand your question correctly you have an external HDD, presumably with a USB connection, and you want to get a computer that happens to have Windows installed to boot the Ubuntu OS on the USB drive. You certainly don’t do that from Windows. You need to get into your BIOS/UEFI and change the boot order. If you make USB the first boot device it will boot that when it’s connected and boot Windows when it’s not. You will need to turn off secure boot if you have a modern system with that enabled.