Is this what you are going for?
(defun my/create-new-note ()
"Create a new note file in `org-roam-directory'."
(interactive)
(let* ((time-string (format-time-string "%Y%m%dT%H%M%S"))
(extension "org")
(file-base-name (concat time-string "." extension))
(file-name (expand-file-name file-base-name org-roam-directory)))
(find-file file-name)
;; In the new buffer
(insert (format "#+title: %s\n\n" time-string))
(org-id-get-create)
(end-of-buffer)))