Remove the login control and add 2 TextBoxes for input and a Button.
And assign the OnClick event handler to the Button and call the method ValidateUser
<asp:Button Text = "Login" OnClick = "ValidateUser" />
Then in ValidateUser method, replace
cmd.Parameters.AddWithValue("@Username", Login1.UserName);
cmd.Parameters.AddWithValue("@Password", Login1.Password);
with
cmd.Parameters.AddWithValue("@Username", txtUserName.Text);
cmd.Parameters.AddWithValue("@Password", txtPassword.Text);