Normally, if you have the following in /etc/default/grub… 
GRUB_DISABLE_OS_PROBER=false
… then Windows should be added to the GRUB boot menu. If it is set to true, then change it to false and run…
sudo update-grub
Also make sure that you have the following line near the top of /etc/default/grub before saving the file.
GRUB_TIMEOUT_STYLE=menu