Open (Show) BootStrap Modal Popup during registration in ASP.Net using VB.Net

Refer below code.

HTML

<script type="text/javascript" src='https://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.8.3.min.js'></script>
<script type="text/javascript" src='https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/js/bootstrap.min.js'></script>
<link rel="stylesheet" href='https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/css/bootstrap.min.css'
    media="screen" />
<form id="form1" runat="server">
    <div style="max-width: 400px;">
        <h2 class="form-signin-heading">Registration</h2>
        <asp:Label Text="Username" runat="server" AssociatedControlID="txtUsername" />
        <asp:TextBox ID="txtUsername" runat="server" CssClass="form-control" placeholder="Enter Username" required="required" />
        <br />
        <asp:Label Text="Password" runat="server" AssociatedControlID="txtPassword" />
        <asp:TextBox ID="txtPassword" runat="server" TextMode="Password" ToolTip="Password must contain: Minimum 8 characters at-least 1 Alphabet and 1 Number"
            CssClass="form-control" placeholder="Enter Password" required="required" pattern="^(?=.*[A-Za-z])(?=.*\d)[A-Za-z\d]{8,}$" />
        <br />
        <asp:Label Text="Confirm Password" runat="server" AssociatedControlID="txtConfirmPassword" />
        <asp:TextBox ID="txtConfirmPassword" runat="server" TextMode="Password" CssClass="form-control" placeholder="Confirm Password" required="required" />
        <br />
        <asp:Label Text="Email" runat="server" AssociatedControlID="txtEmail" />
        <asp:TextBox ID="txtEmail" runat="server" TextMode="Email" CssClass="form-control" placeholder="Enter Email" required="required" />
        <hr />
        <asp:Button ID="btnSignup" runat="server" Text="Sign up" CssClass="btn btn-primary" OnClick="OnSignup" />
    </div>
</form>
<script type="text/javascript">
    window.onload = function () {
        var txtPassword = document.getElementById("<%=txtPassword.ClientID%>");
        var txtConfirmPassword = document.getElementById("<%=txtConfirmPassword.ClientID%>");
        txtPassword.onchange = ConfirmPassword;
        txtConfirmPassword.onkeyup = ConfirmPassword;
        function ConfirmPassword() {
            txtConfirmPassword.setCustomValidity("");
            if (txtPassword.value != txtConfirmPassword.value) {
                txtConfirmPassword.setCustomValidity("Passwords do not match.");
            }
        }
    }
</script>

Namespaces

using System.Configuration;
using System.Data.SqlClient;

Code

protected void OnSignup(object sender, EventArgs e)
{
    string constring = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constring))
    {
        using (SqlCommand cmd = new SqlCommand("INSERT INTO Users (Username, Password, Email) VALUES (@Username, @Password, @Email)", con))
        {
            cmd.Parameters.AddWithValue("@Username", txtUsername.Text);
            cmd.Parameters.AddWithValue("@Password", txtPassword.Text);
            cmd.Parameters.AddWithValue("@Email", txtEmail.Text);
            con.Open();
            cmd.ExecuteNonQuery();
            con.Close();
        }
    }
}

For more details on registration refer Simple User Registration Form Example in ASP.Net.