I'm a little skeptical about your interest in preparing your own HI, but I don't think my answer is going to help you if you plan to use it for illicit purposes. I answer only to prevent you from blowing your or someone else's head off.
If you really do have a license to purchase HI and the capability and training to make it safely and properly, you should have ample access to resources sufficient to sort this out. I recommend that you DO sort it out before you make another attempt to prepare it.
Your reaction is producing ammonia. Ammonia + HI ---> NH4I. The ammonia comes from the disproportionation of hydrazine - 3 moles of hydrazine produce 1 mole of N2 and 4 moles of NH3.
This reaction is often explosive. The prospect of being showered in scalding hot HI should deter you from trying again.