<button id="mybutton" runat="server" onserverclick="myfunction">
<span style="text-decoration:underline;">P</span>rint</button>
and use a normal button event in your c# code:
protected void myfunction(object sender, EventArgs e)
{
Response.Write("clicked");
}
OR try with AccessKey property
<asp:button id="Button1" runat="server" Text="Print" AccessKey="P" />