You can use almost any type of prepared mix, even potting soil directly from a bag, compost, ground composted pine bark, loamy garden soil, as long as the mixture drains well.
Very important! Soggy soil will cause the cuttings and any root development to decay.

For rooting Hydrangeas, I use a mixture of sand, milled sphagnum peat moss, composted ground bark and perlite. Approximately equal amounts of each. But I have rooted Annabelles in ground composted pine bark alone.

Some people use a clear plastic bag to tent the pot, for moisture retention. I never do, as I have many and it's easy to add water as necessary, when making my rounds in the garden.

If you use the plastic bag, insert 4 sticks in the perimeter of the pot, tall enough to prevent the plastic from contacting the cuttings. Permits air circulation all around the cuttings.

Check for moisture in the rooting medium, at least once per week. You can do this by inserting a forefinger 2-3 inches into the pot. If it feels dry, add enough water to completely moisten it.

Roots should begin to form within 6-8 weeks. Check by tugging gently on each cutting. If there is resistance, then roots have begun to form. Don't be in a hurry to remove them from the potting mix. I usually allow mine to overwinter outdoors and remove them for planting in the spring. At that time, each cutting should have a healthy, substantial root system. I carefully dump the entire pot and then tease the roots apart.

Place the potted cuttings in a full shade location and protect from excessive wind exposure.

Hope this isn't too long winded! I get carried away when talking about plant propagation, one of my favorite gardening exercises!

I'm trying 2 methods on Nikko cuttings as well as ground layering - both look promising with new leaf growths! €;)

Good luck!