I'm scared to tell you how to do what you are asking.
It's not something the average person should be messing with.. and requires some advanced computer knowledge.
Especially when it comes to Win10 which I have to assume you are using.
So can I steer you into a safer/easier/better way to get the same result.
1. Buy this:
https://www.amazon.com/Kingwin-Optimized-Controls-Provide-Longevity/dp/B00TZR3E70?th=1(Kingwin Hard Drive Power Switch Module)
2. Set your boot order in the bios. "Gaming" drive Hd2 to load 1st. Default drive "Hd1" to load 2nd.
2b. If you have a hot-swap option in your bios, turn it on as well. But it's not necessary.
3. Install the switch & connect your 'gaming' drive to the Kingwin device.
4. Any time you want to game, just press the 'on' button before you power the system on.
5. Done. And you didn't brick Windows by messing with the bootloader.
---
If you really want to have the boot menu option, start studying here:
https://www.boyans.net/