Hi there!
In, general, to
register an application with UltiDev Cassini you need to provide its location (physical folder), default document (like Default.aspx or index.htm) and application ID. When you use "Pick Default Document" button, you can select the default document file, and then hit Generate button, which will create an application ID and populate both Physical Location and Default Document fields. After that you click Save button, and your application is going to be hosted by UltiDev Cassini.
One word of caution, though: UltiDev Cassini is a web server designed mostly for intranet (local area network) hosting, not Internet hosting. We recommend considering security implications very seriously when hosting Internet-facing sites under UltiDev Cassini.
Please let us know if this information was helpful.
Best regards,
UltiDev Team.
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http://www.ultidev.com/products/Donate.aspx to help us improve our products.