How to change the Postbackurl path of an Asp. Button?

Question

I have the following situation: on my page . aspx I have an Asp.Button that, after running an Onclick event on Behind, runs a Postbackurl to another page carrying information through Hiddenfields.

What I am trying to do is: when trying to register a client, if the email entered is already registered in the database, the button does not run the Postbackurl or change its URL so that the page does not change. Otherwise, run normally and proceed to the profile.

My button on the front :

<asp:Button ID="continuebtn" OnClick="Continuebtn_Click" runat="server" PostBackUrl="~/cliente/perfil.aspx" CssClass="btn btn-success btn-sm" Text="Continuar"/>

And the Behind code looks like this (briefly) :

protected void Continuebtn_Click(object sender, EventArgs e)
{
    using(MySqlConnection conn = new MySqlConnection(conexao))
    {
        try
        {
            string consulta = "SELECT * FROM tb_cliente WHERE Email = @email"
            MySqlCommand cmd = new MySqlCommand(consulta,conn);
            cmd.Parameter.AddWithValue("@email",clienteEmail);
            MySqlDataReader reader = cmd.ExecuteReader();
            if(reader.Read())//caso exista um cliente cadastrado com o email inserido
                //altera o PostBackUrl para se manter na mesma página
            else//se o email inserido for novo
                //mantém o PostBackUrl do botão
        }
        catch (MySqlException ex)
        {
            EscreveLog(ex.Message);
        }
        finnaly
        {
            conn.Close();
            reader.Close();
        }
    }

}

So far I’ve tried :

continuebtn.PostBackUrl = "";

continuebtn.Attributes["PostBackUrl"] = "return false";

continuebtn.Attributes.Remove("OnClick");
continuebtn.Attributes.Add("OnClick","return false");

continuebtn.OnClientClick = "return false;";

colocar só um return no else

And no alternative worked, all continue to run Postback with the url "~/client/profile.aspx"

I hope I’ve been clear on the explanation, any help is welcome.