Installed Parrot but can't boot into it

I’ve installed Parrot linux 4.1 in a 300GB partition on a system that has Windows 10 on. During the install the GRUB installer failed to detect Windows, so I chose not to install it for fear of losing access to windows.

So at the moment I can only boot into Windows, what I want to do is boot into linux when I want by inserting a USB stick and getting it to boot into linux instead of windows when required. I’ve tried installing GRUB onto the USB by following the instructions here:

yet I’ve tried all 3 configs and it won’t show as bootable from the list when I press F12 (other USBs can and it’s formatted as FAT32) - I have disbled UEFI so I get the option to boot legacy

Please can someone advise how I can boot into my parrot partition via USB instead of installing GRUB onto my main hard drive proper?