How can Windows be installed to a Mac via Boot Camp in BIOS Boot mode?
Assuming you are using the Boot Camp Assistant to install a version of Windows officially supported on your Mac, then Windows will be installed to BIOS or UEFI boot automatically without user interaction.

During the Boot Camp installation process, Windows determines the boot method based on how the Boot Camp Assistant partitions the installation drive. If Boot Camp Assistant choose to keep a GUID partition table (GPT) with a fully protected Master Boot Record (MBR) partition table, then Windows will install for a UEFI boot. The alternative is to change to a GPT and a partially protected MBR partition with entries for the macOS (OS X) and Windows partitions. This is referred to as hybrid partitioning. In this case, Windows will install to BIOS boot. The older Macs only support hybrid partitioning for Windows installations, because there is a BIOS, but not a new enough EFI version in the firmware. Newer Macs support both methods of partitioning for Windows installations, because there is both a BIOS and new enough EFI version in the firmware. However, the newest Macs only support a GPT with a full protected MBR for Windows installations, because there is no BIOS support and enough EFI support in the firmware.