I'm trying to setup windows 10 on a flash drive from my macbook using the Boot camp assistant. It recognizes my windows 10 iso file, I am only given one option of using it on disk1 of my own computer and cannot change this to my flash drive. How can I give Boot camp assistant the ability to recognize the external drive?

I'm running High Sierra 10.13.2

I'm trying to setup and iso file on a flash drive from my macbook using the Boot camp assistant. It recognizes the iso file, but I am only given one option of using it on disk1 of my own computer and cannot change this to my flash drive. How can I give Boot camp assistant the ability to recognize the external drive?

I'm running High Sierra 10.13.2

For reference, the iso file is for the windows 10 os.