You need to do it via the brain.
A muscle doesn't know if it's involuntary or voluntary - the biology of these muscles (and the nerves) are essentially the same, there's no known chemical that will selectively target one but not the other.
The distinction is fairly poor too - I have many involuntary muscles; coughing when dust gets in my lungs, gag reflex, pupil dilating, shivering when cold, recoiling from pain, and then these "involuntary muscles" can be controlled - I can hold my breath, not to mention people can override their gag reflex, etc.
So you need to turn off someone's higher brain function and just leave their lower functions working. As a society we know a way to do this already.
200cc's of ethanol should do the trick. Administer orally in an 8% solution with soda.
Many other recreational substances can cause similar effects in sufficient concentration.
I'm also not game to google "What's the best date rape drug?" but suspect that could also answer your question.