If you examine the demo's scripts you will see that the value is used to obtain the string that is used as the URL for RadWIndow. Note that this must be a client-side resolvable URL, one you could type in the browser and have it work - either an absolute, or a relative path is needed. The tilde symbol (~) is used to represen the site root on the server and the browser cannot read it. You could replace it with a slash (/) on the client in most cases. Another reason why the Value is used is to prevent the default action of a menu with a NavigateUrl - taking the browser to this page. If you are building your menus in the code-behind you can concatenate the URL with ResolveUrl("~/"); to get the root.
Said shortly, you would need an URL in the following format:
because the browser cannot resolve "~/myFirstPage.aspx" and thus, neither can RadWindow. This works in server controls that direcrly use this because they resolve it internally (see the ResolveUrl() method) which is not the case here.
the Telerik team
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