Related to How to associate application with existing file types using WiX installer? and How to register file types/extensions with a WiX installer?.
How I can associate application with existing file types if the installation is per-user?
No HKLM key allowed
I believe this is not really different from the per-Machine case. For downmarker (which is installed per-User) I put this in the component which installs the application executable:
Or you can look at the full wxs file.