The payload fairing has holes, it's not hermetically sealed. The air escapes as the rocket ascends. It's a windshield, not a pressure vessel. Falcon 9 reaches above breathable air seconds after launch.

200kg capacity so you could build a tiny space capsule with enough air to keep you alive for a few hours. Getting back to Earth alive is an exercise left up to the reader.

You'll also have to fit yourself (and your spacecraft) into a 24"x24"x24" space so I suggest consulting with some contortionists and losing some weight. Do you really need legs?

So how long before the first space stowaway? Maybe when launches become 100x or 1000x more common, one firm will have lax enough security that some teenager can sneak aboard in a homemade spaceship or spacesuit.

100kg to space is a _lot_ of fuel, I think they'd notice. I'd be shocked if weight isn't checked on the launch pad.