You can do it by using javascript.
Say your popup page name is 'ForgotPassword.aspx'
and your button id is 'btnGo'
<script language="javascript">
function OpenForgotPassword()
{
strUrl="ForgotPassword.aspx";
x = window.showModalDialog(strUrl,"newwin","dialogHeight:20;dialogWidth:36; status:No; edge:Raised; center:yes; help:No; resizable:No; ")
}
</script>
.aspx.cs page coding
protected void Page_Load(object sender, EventArgs e)
{
btnGo.Attributes.Add("onclick","return OpenForgotPassword();");
}