Hello dude use this code.
<form id="form1" runat="server">
<div>
<asp:scriptmanager id="sc" runat="server" xmlns:asp="#unknown">
</asp:scriptmanager>
<asp:updatepanel runat="server" id="uPanel" xmlns:asp="#unknown">
<contenttemplate>
<asp:panel runat="server" id="pnlEtryForm">
<table>
<tr>
<td>
</td>
<td>
</td>
</tr>
<tr>
<td colspan="2">
<asp:button runat="server" id="btn" text="test" onclick="btn_click" />
</td>
</tr>
</table>
</asp:panel>
<div id="pnlFormDonate" runat="server" visible="false">
<form>
</form>
<form id="frmDonate" action="https://www.sandbox.paypal.com/cgi-bin/webscr" method="post">
<input class="empty" type="hidden" id="bus" name="business" value="kanag._1336399919_per@gmail.com" />
<input type="hidden" name="cmd" value="_donations" />
<input type="hidden" name="item_name" value="Donation For IJP" />
<input type="hidden" id="itemNo" name="item_number" value="Donate for IJP Ministry" />
<input type="hidden" name="amount" id="amt" />
<input id="Image1" runat="server" type="image" src="https://www.paypalobjects.com/en_US/i/btn/btn_donateCC_LG.gif">
border="0" onclick="Submitfrom()" name="submit" alt="PayPal - The safer, easier way to pay online!" />
<img alt="" border="0" width="1" height="1" src="https://www.paypal.com/en_US/i/scr/pixel.gif" />
</input></form>
</div>
</contenttemplate>
</asp:updatepanel>
</div>
</form>