Often, partitions containing Windows will be automatically discovered by
os-prober. However, NTFS partitions may not always be detected when mounted with the default Linux drivers. If GRUB is not detecting it, try installing
NTFS-3G and remounting.
Encrypted Windows partitions may need to be decrypted before mounting. For BitLocker, this can be done with
dislockerAUR. This should be sufficient for
os-prober to add the correct entry.