When installing linux along with windows you ll need to add windows boot entry to grub.
Follow the steps below to add windows menu in grub boot menu.
Install the os-prober, os-prober helps grub-mkconfig search for other installed systems and automatically add them to the menu.
# pacman -S os-prober
Mount the file system where windows is installed, for example windows 10 is installed on /dev/sda3
# mkdir /mnt/windows10 # mount /dev/sda3 /mnt/windows10
Update grub config
Reboot the system, you ll see windows entry in grub boot menu.