Adding my own form

Hi, i want to add a wpform on my maintenance page. I have the pro version too. I can add the shortcode in the html section but the shortcode doesn’t do anything. It just shows the shortcode. How can i be able to add my own contact form ?

Second of all, How can i change the height of my page because when i put in middle center to be in middle of the window, nothing change. All the contents are stick to the top and are not in the middle of the page.