You can't, they're a private company. Can't short Uber either.
edit: maybe its more obvious in a different industry: assuming a hurricane wrecks florida. now you want to profit from shorting that event, so you short disaster insurance providers because you know they will have to pay for the damages. but the public market was faster. the losses are already priced into the stock, although those losses havent been realized yet. you can short it all you want, but you will be "too late" with your "unique insight".
There is a lot of hype in media, everybody is seeking new unicorn to invest in, so it might be the case that all the funds managers are still in denial about the true worth of technology companies.
It wouldn't be uprecedented, in fact current market situation looks a lot like just before dot-com bubble bust: 8 years of bull market, and a lot of money invested in companies without viable business model, like Uber.
you seem to lack a solid grasp of economics. the reason uber is "losing money" is because it is competing in an oligopoly and tries to get rid of its competition. the reason it is being funded to "lose" billions of dollars every year is because it is clearly valuable, not because some "hedge fund idiots" dont know what to do with their money.
once uber turns into a quasi monopoly, it will be able to fix its pricing structure. if it goes too far, it will be regulated. uber is already more of a public utility than "just a company".
calling something that millions of people love "not viable" has never worked. and never will. uber and airbnb are solving the major problem of "you have to own a house and a car". thats a major achievement and costs a lot of money to establish. especially due to regulation trying to prevent people from becoming more mobile and less enslaved.