If you want to open the Command Prompt window on your Login screen or the Boot screen in Windows, here are different ways to do it.

With the Terminal, Windows has a common interface for Powershell, Command Prompt, and Azure Cloud Shell. This allows you to start the terminal directly with the command prompt.

Open a universal command prompt in the folder with the files you want to compare. Type fc textfile1.txt textfile2.txt into the command window, then press Enter.