typed/racket: Declare struct in unit signature

Given I have Racket typed module

(require typed/racket/unit
         Engine/engine2-sig)

(define-unit engine2@
  (import)
  (export engine2-sig^)

  (define-struct posn ([x : Natural] [y : Natural] [z : Natural])
    #:transparent)
  )

(provide engine2@)

with following signature

#lang typed/racket

(require typed/racket/unit)

(define-signature engine2-sig^
  ([posn : ???]))

(provide engine2-sig^)

What should I put instead of question marks to export struct called posn?

I cannot find appropriate example in docs here, am I missing something?