<asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
    <input id="Button1" type="button" value="button" onclick="Email()"/>
    <input id="txt" type="text" />

<script>
        function Email() {
            var strEmails = document.getElementById('<%=Label1.ClientID %>').value;
            var txt = document.getElementById('txt');
            txt.setAttribute('value', strEmails);
        }
    </script>