I use rEFInd, which auto-detects my Windows boot partition. Though I had the Windows installation before the Linux one.
Systemd-boot should be able to detect a bootable Windows too. I don’t remember the specifics. It works out of the box on my laptop. Google “systemd Windows boot”.
Alternatively, maybe something went wrong with the Windows install and you nees to do the Windows repair USB stick thingy. Do you have a Windows recovery partition?