Fire Button event in Page_Load

This should be an easy solution but I get stuck with it and hope I can get some advice/guidance from you:
I have a simple aspx page with a Paypal button in it.   The code shows in .aspx page:

<form action="https://www.paypal.com/cgi-bin/webscr" method="post" id="PayPalForm" name="PayPalForm" runat="server">
<asp:HiddenField runat="server" ID="cmd" Value="_s-xclick" />
<asp:HiddenField runat="server" ID="hosted_button_id" Value="xxxxxxx" />
<asp:ImageButton runat="server" ID="imgDonate" ImageUrl="https://www.paypal.com/en_US/i/btn/btn_donateCC_LG.gif"
AlternateText="PayPal - The safer, easier way to pay online!"
OnClick="imgDonate_Click" /> ...