Hello Kalyan,
As far as I know there is no way to work around that restriction for the browser enabled forms. You simply can't add the System.Windows.Forms.dll. I had the same problem as you and the only solution I could think of was to add a hidden section in the form and show it when required.
If somebody else knows a better way of solving this, please, don't be shy :-)
Hugs,
Daniel Rivas.