Using MSIX, how do I show a EULA

The recommendation from Microsoft is to move all the configurations you would perform in custom dialogs in your application, as the first-launch experience for your users.

So the first time your users launch the application you will show them the EULA in a custom dialog from your app, along with other options that your application might require to be configured by the user before it starts using it.

Also, make sure you save any user options in AppData and not in config files found next to the application binaries, in the installation folders (you don't have write permissions there). I know this advice might sound silly, but I've seen this problem so many times.