An example of getting user input for an Elisp function with a default value divined from context.

input_example.el

(defun input-example (text)
  "Gets user input and prints it back. Defaults to word at point"
  (interactive
   (list 
    (read-string 
     (format "Text (%s): " (word-at-point)) ; prompt. It's nice to show the default value
     nil ; initial input.  This value is prefilled in the mini-buffer. Available but considered deprecated.
     nil ; history list if you have a specific one for this
     (word-at-point) ; Whatever this evaluates to will be the default value
     )))
  (message (format "You said '%s'" text)))