I have a standard asp:login control:
In the designer of your Login control: "Convert To Template". Then in the Page Load set the defaultButton of your form by finding the LoginButton.
ASPX:
Code-Behind:
protected void Page_Load(object sender, EventArgs e)
{
Button lbButton = Login1.FindControl("LoginButton") as Button;
form1.DefaultButton = lbButton.UniqueID;
}