There's a hollow peg-esque type thing protruding near the centre top (above where the normal null rays can be stored). The crown popped out the first few times I tried but stayed put thereafter. It's also illustrated in the instructions.
My nickname for this guy is Barbie-starscream and I mean this in the nicest way possible. I do hope the popularity of Coronation Starscream paves the way for more accessory-laden releases. With the rising cost of everything this could be a not-so-expensive way for Hasbro to bring value to consumers as RRP inflates.