tick the checkbox on to Operating System under Add to USB disk subheading.
For example, I am using the Windows 8.0 ISO.
Note: If the size of your ISO is larger than 4 Gb,
it will show a message to split the file into parts. This is because you’ve chosen the FAT32 option.
WinSetupFromUSB doesn’t support dual ISOs i.e. it can’t have 32-bit and 64bit version in a single ISO. It will show an error message so be careful.
Click GO. A data deletion warning message appear on screen. This is because you’ve chosen to format the flash drive. Click Yes.
partitions will be erased. Click Yes.
It will ask for the folder name. Type the desired one in 30 seconds or it’ll auto select. Click Ok.
It will ask you for the boot menu name.
This will appear when you run the multiboot USB on some PC and choose an operating system.
Type your choosen name like Windows 8.1 64-bit. Click Ok.
The processing will done in few minutes.
Click EXIT to finish.
How to add the second ISO file in the Multiboot USB Disk
Start the tool again.
Select your flash drive in the drop-down menu.
Click Advanced Options and look for Custom menu names for Vista/7/8/10/Server Source.
Don’t Click Auto format with FBinst. Because if you clicked then it will remove your previous ISO file.
Add the second ISO file for your multiboot USB.
Click GO and repeat the steps mentioned earlier.
Follow the same process to add more ISO files.