Hi Oman,
Probably the best medium for cuttings would be something like, John Innes number 1or Arthur Bowers seed compost, it needs to be free draining which can be improved by adding horticultral sand.
I think the main secret of success though is to provide bottom heat for them to strike well using semi-ripe wood cuttings, otherwise layering is a good way of propagating hibiscus. Good Luck Tbird+