I am not able to reproduce this on my end - see video and let me know if I am missing something. You can also add it in the HTML included in the <head> tag (except title, keywords, description) field under the Advanced section - video. Additionally you can go through our Documentation for more information on creating pages.
Can you tell me which is the exact Sitefinity version you are using. In order to do that go to your project's->bin folder and locate the Telerik.Sitefinity.dll. Right-click it and select Properties. Go to the Details tab where you can see the file version.