The topic of saving has been discussed endlessly here. Unfortunately, all easy solutions depend on the user's platform and software.
The universal solution is to configure the browser. Oh come on, users can handle that, can't they?
Nearly universal solution: Put the TW file on tiddlyspace and give the user the password. Downside: The file is publicly viewable.
For windows, there are at least two solutions that come close to requirements:
On Windows, there is a way to wrap a TW file in it's own executable wrapper. See
https://ihm4u.github.io/twexe. This comes closest to the "dream" solution. But some people warn that this approach may be dangerous. Hey, but YOLO, right? Also leaves a command box open, which user must be smart enough not to close.
On Windows, you can change the extension of your tw file to .hta and then open/save in Internet Explorer. Unsure if it works in either version of MS Edge. Maybe someone running Windows 10 could test that for you.
HTH