Validate TextBox value inside Bootstrap Modal Popup in ASP.Net using C# and VB.Net

Hi ramco1917,

Please refer below smaple.

HTML

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.6.1/dist/css/bootstrap.min.css" />
<script src="https://cdn.jsdelivr.net/npm/jquery@3.6.0/dist/jquery.slim.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/popper.js@1.16.1/dist/umd/popper.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/bootstrap@4.6.1/dist/js/bootstrap.bundle.min.js"></script>
<div class="modal-dialog">
<div class="modal-content">
    <div id="modal_form_horizontal">
        <div class="modal-header bg-info">
            <h5 class="modal-title">Add/Update Record</h5>
            <button type="button" class="close" data-dismiss="modal">&times;</button>&nbsp;                                   
        </div>
        <div class="form-horizontal">
            <div class="modal-body">
                <div class="row">
                    <div class="col-lg-3">
                        <div class="form-group">
                            <label>Year</label>&nbsp;<span style="color: red">*</span>
                            <asp:DropDownList ID="ddlYear" class="form-control" runat="server">
                                <asp:ListItem Value="">Select Year</asp:ListItem>
                                <asp:ListItem Value="">2012</asp:ListItem>
                                <asp:ListItem Value="">2013</asp:ListItem>
                            </asp:DropDownList>
                        </div>
                    </div>
                    <div class="col-lg-6">
                        <div class="form-group">
                            <label>Rating</label>
                            <asp:TextBox ID="txtRating" class="form-control" numaceholder="Rating" required="true" runat="server"></asp:TextBox>
                            <asp:Label ID="lblMessage" Text="Please enter 100 only." runat="server" Visible="false" ForeColor="Red" />
                        </div>
                    </div>
                </div> 
            </div>
            <div class="modal-footer">
                <button type="button" class="btn btn-link" data-dismiss="modal">Close</button>
                <asp:Button ID="btnSave" runat="server" class="btn bg-primary" Text="Save" OnClick="Validate" />
            </div>
        </div>
    </div>
</div>
</div>

Code

C#

protected void Validate(object sender, EventArgs e)
{
lblMessage.Visible = false;
if (txtRating.Text != "100")
{
    lblMessage.Visible = true;
    string message = "The Value shoud be 100... ";
    ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('" + message + "');", true);
}
}

VB.Net

Protected Sub Validate(ByVal sender As Object, ByVal e As EventArgs)
    lblMessage.Visible = False

    If txtRating.Text <> "100" Then
        lblMessage.Visible = True
        Dim message As String = "The Value shoud be 100... "
        ClientScript.RegisterStartupScript(Me.GetType(), "alert", "alert('" & message & "');", True)
    End If
End Sub

Screenshot