post xml data to asp.net page with username and password

Form a query string for login.aspx page. Like this

YourSite/Login.aspx?xml=xmldocument

Then detect the query string in page load or oninit of Login.aspx using,

var loginCredentials=Page.Request.QueryString["xml"];

Then parse the xml nodes to get the username and password and then authenticate it using any type of authentication method.