I've just read the topic in the Innosetup forum. The help given there is a bit "short". But rather than you asking for help in the XnView forum you should continue in the Innosetup forum.
From what I get out of the discussion, using "cmd.exe" for writing a text file is possible, but not really the way to go. Your solution might work on your Windows version, but might cause problems on other Windows version with a different command line interpreter. There might be security issues, too. Last not least Pascal scripting is not too hard to learn. These are probably the reasons why the Innosetup people do not want to support your solution in any way.
Ask the Innosetup people there for a Pascal example that writes a text file (that should be a piece of cake for them) and for the requisites (Compiler, ...) needed. Then, you can modify the example, compile and run it. It's a bit harder way to go, but I think it's worth the effort, because you will have a reliable setup program, then.
thanks for your reply. At the moment, I stopped my idea with the txt-file, because I made "only" a company-internal re-packaging of an external software and the txt would be only an additional, but not absolutely necessary feature.