Microsoft added a custom UI option for MSIX Packages several years back, but the only reference to it was a web page How to create a custom App Installer UX – MSIX | Microsoft Learn that contained the following notice:

Because of this note, we have been ignoring this capability. I recently detected that the page had been updated, although …
TMEditX is the licensed and more capable tool for fixing up MSIX packages since it doesn’t have to depend on the Microsoft MSIX Packaging Tool to create the final package.
The wicked cool open-source application Notepad++, a free XML editing tool, is an example of an app commonly deployed in enterprises that not only has a shortcut to start the application, but also uses the Windows file explorer File Type Associations to allow the end user to launch Notepad++ from any file on the system by right-clicking.
TMEditX is the licensed and more capable tool for fixing up MSIX packages since it doesn’t have to depend on the Microsoft MSIX Packaging Tool to create the final package.
PsfTooling is the free tool to work in conjunction with the Microsoft MSIX Packaging Tool.
The Package Support Framework (PSF) was originally an open-source project to help Win32/DotNet applications run when inside an MSIX Container. TMurgent has taken up the support for the PSF in our own fork