Mac Pro starts with folder icon and question mark, HD is empty. How to install Ubuntu? I have a Mac Pro with an empty (no existing partitions) 1TB HD connected to it.
I am using a windows USB keyboard and want to install Ubuntu on it.
There is no Mac OS installed or running on this computer. I have no access to another Apple device.
It is not a Macbook.
This is the exactly version: Mac Pro 3.00x/4x1/x1900xt/250/sd/ap
Questions:

how to boot the Mac Pro using a USB stick and install Ubuntu on it?
can I do it using a simple USB keyboard and Microsoft Wireless mouse?
I suggest you use the tutorial from Ubuntu.
This requires access to another running computer (Linux, Windows, Mac). If you don't have one, consider to reinstall macOS with macOS Recovery installer to prepare the stick. Press shift + opt + cmd + R during a restart to reinstall the macOS that came with your Mac, or the closest version still available.