The firmware, stored on the logic board of your Mac, contains both BIOS and UEFI boot loaders. Additionally, the firmware on the logic board contains the Mac Startup Manager, which allows you to choose available startup disks or volumes. You can access the Mac Startup Manager by holding down the Option (⌥) or Alt key immediately after starting or restarting your Mac.

IFAIK, the the Mac Startup Manager can not be configured to to ignore the internal drive.

However, there exists other startup (boot) managers which can be configured to ignore the internal drive. One such free product is the rEFInd Boot Manager. To configure rEFInd to only search for BIOS booting external drives, one only has to add the following line to the end of the refind.conf file.

scanfor biosexternal

Furthermore, the rEFInd Boot Manager can be configured during installation to be accessible from the Mac Startup Manager. The rEFInd Startup Manager can be installed in your existing EFI partition, so you will not have to make changes to your external drive. If desired, rEFInd can silently BIOS boot windows installed on an external drive.

I used a 2013 iMac to verify your problem with the Mac Startup Manager exists. Next, I installed rEFInd to verify rEFInd would solve the problem. A more detailed explanation is given as an answer to the question Two Bootcamp installations: first one on Internal HDD and second one on external Thunderbolt 2 SSD.