Thanks, but what am I doing wrong here:

$(document).ready(function () {
            jQuery("#MainAspNetForm").validationEngine();
            $('TextBox_UserMessage').rules('add', {
                required: true,
                messages: {
                    required: 'Some custom message for the required field'
                }
            });
        });

...

<asp:TextBox ID="TextBox_UserMessage" runat="server" TextMode="MultiLine" Rows="7" ClientIDMode="Static"
                Width="300" class="ContactUs"></asp:TextBox>