eww!, just my two cents here.... They make a A/c smell elimationor that you inject into the evap box to kill any fungus growing in there from the moisture. pouring anything into the windshield cowl will do nothing but mask the smell. To really get rid of it you either use the stuff i'm talking about, or evac the a/c, pull the evap box, clean out the evaporator and replace the evap foam, then put it back together and recharge the a/c.
The cowl and the evap box are not connected..... whoever tells you pouring something in the cowl will work isn't familar will a honda evac system. it will MASK it if you are lucky but you can't fix it this way.
Try a search on google for this stuff I'm taking about, the company that we buy ours from is called MOC. It seems to work good. You simply drill a small hole into the evap box, inject the stuff, then fill the hole you just made with tape, epoxy, sealer..