How to install bootloader on sdcard on install

Hi, i use the hp microserver gen8 in ahci mode with OS hdd on sata port 5. Normaly it wont boot from that port in ahci mode, while more then one hdd is attached.

to override that isue i put an sd-card on internal sd-port and put bootloader on it.
Normaly i use debian.
In debian i can select on witch device the bootloader is to install.
How i do it in rancheOS

Sorry for my bad english, im german.