<asp:DetailsView ID="DetailsView1" runat="server">
        <Fields>
            <asp:TemplateField HeaderText="form_c" SortExpression="form_c">
                <EditItemTemplate>
                    <asp:Label ID="Label1" runat="server" Text='<%# Eval("form_c") %>'></asp:Label>
                </EditItemTemplate>
                <InsertItemTemplate>
                    
                // вот этот элемент
                    <asp:TextBox ID="TextBox1" onchange="testcheck();" runat="server" Text='<%#            Bind("form_c") %>'></asp:TextBox>

                </InsertItemTemplate>
                <ItemTemplate>
                    <asp:Label ID="Label1" runat="server" Text='<%# Bind("form_c") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
....................

function testcheck()
    {
        var txt = document.getElementById("TextBox1");
    }

DetailsView1_TextBox1