You can add a video to any page of your webapp by using the tool box.
First you need to open the webapp you are working on then click edit pages. Now click the edit icon of the page where you want to add a video.
On the left under Tool Box look for the YouTube icon and click it.
A pop-up page will open where you will paste the embed code of the video. You will need to change the size of the video. The maximum size is 320 x 265 but you can make it smaller if you want. After changing the height and width click Insert tab.
Below is how it looks on your webapp. If you don't want it to be on top of the page you can easily move it to either below the logo or at the bottom of the page or anywhere else you wanted it to go.
To move your video click HTML found either below or at the top of the iphone. Highlight the code of the video then cut it (Ctrl+x). Scroll down to the desired space where you want it to go then paste (Ctrl+v) the code.
Image below, I have moved the video code below the logo and have changed the size of the image to make it smaller. To have the video show at the center just add <center> before the code and </center> after the code.
Here's the outcome of the changes made. Please don't forget to click the "Save Changes" button and then "Done".