I've not had a t turn me down when I directly asked for a hug, but I usually don't ask for them. I've actually had several t's offer a hug after difficult sessions, or at the termination session. There's only been maybe 2 hugs I've refused, though I did express to the t that I might appreciate one at a later time, just not when I was so triggered.

I'm not very good at asking for what I need verbally, so like someone else mentioned, writing it down is easier. You could also mention not having much supportive physical contact in life and wondering if t would be ok giving you a hug... or, if you are feeling brave, just ask for a hug without explanation unless t asks...