That would be a very bad idea when the system is installed in EFI mode as you will not be able to boot the system.
If you existing Windows is MBR - Manjaro must be MBR too - don’t try to mix EFI and MBR - very bad idea.
You can verify running
sudo fdisk -l /dev/sda
If the disk partition layout is DOS then it is MBR and you should disable EFI in firmware prior to install as doing so will ensure Manjaro will install as MBR.