I would like to assing the function `org-timer-start` with a univeral argument (`Ctrl-u`) to its own key. 

I read [here][1], that one should look into the source code of a function to find out how to pass a universal argument.

Where is this part in the following function, that tells me about the universal argument call?

    (defun org-timer-start (&optional offset)
      "Set the starting time for the relative timer to now.
    When called with prefix argument OFFSET, prompt the user for an offset time,
    with the default taken from a timer stamp at point, if any.
    If OFFSET is a string or an integer, it is directly taken to be the offset
    without user interaction.
    When called with a double prefix arg, all timer strings in the active
    region will be shifted by a specific amount.  You will be prompted for
    the amount, with the default to make the first timer string in
    the region 0:00:00."
      (interactive "P")
      (cond
       ((equal offset '(16))
        (call-interactively 'org-timer-change-times-in-region))
       (org-timer-countdown-timer
        (user-error "Countdown timer is running.  Cancel first"))
       (t
        (let (delta def s)
          (if (not offset)
    	  (setq org-timer-start-time (current-time))
    	(cond
    	 ((integerp offset) (setq delta offset))
    	 ((stringp offset) (setq delta (org-timer-hms-to-secs offset)))
    	 (t
    	  (setq def (if (org-in-regexp org-timer-re)
    			(match-string 0)
    		      "0:00:00")
    		s (read-string
    		   (format "Restart timer with offset [%s]: " def)))
    	  (unless (string-match "\\S-" s) (setq s def))
    	  (setq delta (org-timer-hms-to-secs (org-timer-fix-incomplete s)))))
    	(setq org-timer-start-time (org-time-since delta)))
          (setq org-timer-pause-time nil)
          (org-timer-set-mode-line 'on)
          (message "Timer start time set to %s, current value is %s"
    	       (format-time-string "%T" org-timer-start-time)
    	       (org-timer-secs-to-hms (or delta 0)))
          (run-hooks 'org-timer-start-hook)))))


Do I need to make a custom fuction to call org-timer-start with a `Ctrl-u` prefix?

  [1]: https://emacs.stackexchange.com/questions/14748/how-to-bind-a-command-with-a-c-u-prefix-to-a-different-key