1. Prepend normal text/batch file with "ÿþ"
2. Close and re-open the file.
It's not actually unicode, the only difference is that the byte characters FF FE are a "BOM" or Byte Order Mark. At the start of text file it means the file is unicode and what order the bytes should be expected in. If you open a file starting with a BOM, the editor will figure it's Unicode- so Notepad, and most other text editors, will open your file as if it was.
Command prompt doesn't care about unicode and treats the BOM as whitespace when it interprets it.