@mahesh
The Design Center does not yet support user-managed HTML code. A custom HTML widget is on our todo list.
What you can do if you are comfortable with HTML and advanced CSS is to create the basic structure of a mobile web page. Place a Panel widget where you would like to include your own HTML. Then in the generated HTML you will find an empty inner <div> where the panel’s HTML content can be added manually.
NOTE: Get your UI design exactly how you want it before modifying the generated HTML and CSS. The Design Center will overwrite your customizations of the main HTML and CSS files. Actually the Design Center will ask you if you want to overwrite any files you have manually modified.