If I happen to be already pinning gear that day, i'll just load the hcg in with the rest and shoot intramusc. since I'm already there. If I'm only shooting HCG that day, I use a slin pin and pin sub Q in the stomach. There's no reason to be more invasive than you have to.
The hcg that I got from omega also calls out to inject via IM on the packaging itself. But I personally inject sub-q like everyone else. I beleive if you inject IM it will hit the blood stream faster compared to sub-q.