(defun insert-property(&optional p)
          "insert PROPERTY value of pdftools link"
          (unless p (setq p "TEST"))
          (message "property passed is: %s" p)
          (let ((pvalue
                   (save-window-excursion
                     (message "%s" (org-capture-get :original-buffer))
                     (switch-to-buffer (org-capture-get :original-buffer))
                     (message "retrieved property is: %s" (org-entry-get (point) p))
                     (org-entry-get (point) p)
                     )))
                     pvalue))

I don't think you need any `(insert x)` call for a start: the function itself should not insert the string - it should just return it.

I had a fiddle and came up with the above function, just to satisfy my own curiosity. I hope it helps. I used `org-capture-get` and also `(point)` but maybe that's an alternative way.

Just add `%(insert-property "TEST")%` in my capture template.