How to get the querystring value from the new page that was opened

 Here is the scenario.
I have a URL on a aspx page, say https://ondemand.abc.com/xyz/sessionServlet
Now when I click this url, it navigates to the new page and the url is something like this:
http://localhost/sso/signonServlet?sessionID=38476f646d474b51446c6f3455636b4f39412a2a
I need to capture the sessionID from the above url into a variable.
Basically I have to retrieve the sessionID, and to get that I have to click on https://ondemand.abc.com/xyz/sessionServlet 
All of this should happen behind the scene from clicking to retrieving the sessionID in a variable.
How can I do this, please advice.
 
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