You can't use code editor by default. Its been disabled in WPUF. But, you sure can enable it by modifying WPUF core file. But thats not recommended, Because if there is a new update, You will lose the modification.
Go to WPROOT/wp-content/plugins/wp-user-frontend-pro/class and fine a file named render-form.php. Open it with a Text Editor, And Go to line 729. You will see a codeblock like this,