How to enter special characters in input fields?

The exception you are see has to do with .NETs ValidateRequest check.

You may add the following parameter to disable this check, by setting the check to false: validateRequest="false"  in the main.aspx Page directive.

Warning:

Setting this parameter to 'false' opens you up to potential injection issues. For more information, see the following:
http://msdn.microsoft.com/en-us/library/system.web.configuration.pagessection.validaterequest.aspx