Pre fill minibuffer upon calling to helm-projectile function

Question

Based on this answer, and its with-initial-minibuffer function, I am trying to call helm-projectile-grep, customizing minibuffer pre-filling.

I try to avoid the default behaviour, which is to pre-fill the minibuffer with the contents of the region (if active) or the thing at point.

If I use

(defun with-initial-minibuffer-projectile-grep (str)
  (interactive)
  (funcall `(lambda ()
              (interactive)
              (minibuffer-with-setup-hook
                  (lambda () (insert (format ": %s " ,str)))
                (call-interactively 'helm-projectile-grep)))))

and then I call it with

(with-initial-minibuffer-projectile-grep "mytext")

things go wrong. Any idea of where to start from ?