using interactive for getting into a specific directory

[rustom]

I want to write an interactive function that finds an arbitrary file
from a specific directory "~/org"
The following attempts do not work but should give the idea what I am
trying.

(defun startorg(file)
  (interactive "FFile: ")
  (let ((default-directory "~/org"))
    (find-file file)
  ))

(defun startorg(file)
  (cd "~/org")
  (interactive "FFile: ")
  (find-file file)
)

The general problem can be stated as:
I want to write an interactive command -- here startorg -- whose
interactive actions -- here "F..." -- needs preprocessing with another
function -- here cd.