[Juli174]

с value все правильно.
Мой код:

<div class="param-value">
            <asp:Label ID="txtEmail" runat="server"></asp:Label>
<input id="Button1" type="button" value="button" onclick="splitEmails()" />
            <input id="txt" type="text" value="hello" />
</div>

<script>
                function splitEmails() {
                    var strEmails = document.getElementById('<%=txtEmail.ClientID %>').value;
                    var txt = document.getElementById('txt');
                    txt.value = strEmails;
                }
            </script>