I am trying to add Ubuntu to a new Windows machine. It has a 1 TB SSD with Windows 11 on it. I have shrunk the Windows partition and freed up 500 GB of space for Linux. When I install Ubuntu there's two options "install alongside Windows" or "Something else". Since I want to use another file system (btrfs) I assume I need to choose "Something else" which is what I did. After having assigned the free space for a btrfs / partition and point to the existing EFI partition I did the install. It completed without problems. But when rebooting I was back in Windows with no Linux boot option at all. The Linux partition was shown as "unknown" in Windows.
How can I get dual boot Windows/Ubuntu in this case where I create free space for a btrfs partition to run Ubuntu on?