Assign logged in user name to value attribute of input tag

I am trying to set the value attribute of an input tag to the logged-in user name. The page source code that I would like to have, when the page displays for user JohnDoe, is the following:
        <form action="www.post_to_some_website.com" method="post">                <input id="username" type="hidden" name="username" value="JohnDoe" />                <input type="submit" value="Submit" />        </form>
I have the following ASP.NET code that includes a server side input tag:

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