Lets say I am a german vassal to the carloginian empire and that we are currently under elective. I want the empire to have a gravilkind electorate to ensure that upon the death of Charlemagne I can get my German representation. How would I do this?
You cannot do that as a vassal.
Or by factional demand, which if you have enough supporters might be possible peacefully. If the emperor thinks he can stop you, though, it will be a civil war, so plan ahead.