<form id="form1" runat="server" method="post">
<asp:button id="btnTest" runat="server" text="Test SagePay" onclick="btnTest_click" xmlns:asp="#unknown" />
</form>
and
protected void btnTest_click(object sender, EventArgs e)
{
Response.Redirect("https://test/gateway/test.jsp");
}
Hope it helps.