How do I pass a value from a Windows application to a field on an open web page?
It won't be an easy problem to solve, but I think it is possible by placing the web page in a browser embedded in a .NET application. This Code-Project article can help
Also this article talks a little about accessing the DOM through a C # application.
Do you have any language requirements? And can you add more details on what exactly you are trying to achieve?
EDIT 1: Watij is a web application testing framework for Java. You can use it to fill in text boxes, mouse buttons, etc. I think it might suit your needs, if not open source so you can hack it. There is a whole family of Wati * products - Watin for .NET, Watir for Ruby, etc.
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Gaining access to external web pages is prohibited due to security credentials. But you can open and write to the web page through the winInet APIs. Please see the article
http://www.informit.com/library/content.aspx?b=Visual_C_PlusPlus&seqNum=107
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