Dependency injection in WPF

I'm trying to do DI in my WPF app. I've read about it on different sites and followed youtubers but it doesn't seem to work (it runs but I can't reach the registered service, it'll always be null).
I am basically trying to inject a DbContext as a Singleton because its scoped nature is causing some issues in the application (entity mismatching etc. = badly written code probably).
Help/Tips please!