
I am assuming Windows 10 is staying installed on this computer and you want to be given the choice to boot Windows or Fossapup64?
Yes there is probably a menu entry that could be made for Grub2win.
However, the easy way to go is use and install this version of Grub2 boot loader made to work with Puppy Linux installs.
grub2config-2.0.1
viewtopic.php?t=3360
This is Linux software, so it will have to be run in a Linux OS.
I assume you have Fossapup64 on a USB install you can use to boot the computer using it to run Grub2config.
There is another installer made to do exactly what you want.
Use the Lick installer
https://github.com/noryb009/lick
Lick is a windows exec file you download into Windows, and left click on to run in Windows.
It installs any Puppy Linux OS as a frugal install, alongside Windows, and makes a boot menu entry to boot it.
Computer boots and you get to chose what operating system to boot.
When using Lick.
Not sure what Grub2win did to the normal Windows boot loader.
But you could delete the install of Fossapup64 you did to the C drive or maybe keep it.