In University what is the code that lets sims buy things for their inventory?

not sure what you are talking about, if her hygiene is low just let her shower (or wash hands a couple of times).
The sim's personal inventory works as usual in Uni, you just can't build in the housing dorms if an active sim is already living there, but there are showers in those, so what are you missing exactly?
If you want to build in the housing dorms while living there enter the cheat bb.enablefreebuild, if that is what you are looking for.

I've never heard Sims be able to buy personal hygiene products that will wash a Sim or help them with their hygiene BUT you can buy with aspiration reward points either a potion that will up your Sim hygiene or buy a trait that either will make hygiene go down slower or not at all (depending on the trait you choose). But the most cost-effective way of getting your Sim clean is to throw him or her into the shower. If she or he is in a dorm, the dorm should come with a shower, at least in the default dorms and university housings that comes with the University pack.

@FlamingoKicker1 The cheat to access build/buy mode after implementing testingcheats is bb.enablefreebuild They will need to be in the dorm to use the cheat, of course. Also, your Sim won't be charged for it. So, if they need to account for the price, just use the money cheat to deduct the cost. Hope this is what you needed. :)