<div class="box">
<h1>Вставьте ссылку</h1>
<form method="post">
<input type="text" name="userLink" placeholder="Ссылка..." />
<input type="submit" value="Отправить" />
</form>
</div>
var request = _http.HttpContext.Request;
request.ContentType = "text/html; charset=utf-8";
var form = request.Form;
string l = form["userLink"];