@frakiexx

Use a text editor. Notepad works well to do the job. Open the file in NOTEPAD.EXE or just drag "GMAPSUPP.UNL" file to the Notepad application.
Mark the 25-character code at the bottom field in JetMouse KG and copy with Ctrl+C. Paste in Notepad or other text editor with Ctrl+V (or the lengthy slower method from Edit -> Paste menu selections).

Make sure that you file extension is "UNL" and not "UNL.TXT" when you save the file. Only 25 characters without blanks or carriage return / line feeds.