You will need a Windows PC to run this software initially, OR you can use a Virtual Machine (like Parallels or VMware) on your Mac if you have one already. Alternatively, you can look for a Mac-native alternative like "UUByte Wintousb", but the standard WinToUSB is most reliable.
The Ultimate Guide: How to Install Windows on an External SSD for Mac (Boot Camp on the Go) install windows on external hard drive mac