<asp:TextBox id="SqFtX" runat="server" Width="40px" onkeypress="return isNumber(event,$('#SqFtX').val())"></asp:TextBox>
<asp:TextBox id="SqFtX" runat="server" Width="40px" onkeypress="return isNumber(event,$('#SqFtX').val()); MyFunction();"></asp:TextBox>
<td><asp:TextBox id="SqFt" runat="server" Width="80px" onkeypress="var retval = isNumber(event,$('#SqFtX').val()); if (retval) { MyFunction(); } return retval;"></asp:TextBox>
protected void MyFunction(object sender, EventArgs e)
{
System.Windows.Forms.MessageBox.Show("HERE");
}
<asp:TextBox id="SqFt" runat="server" Width="80px" AutoPostBack="True" onkeypress="return isNumber(event,$('#SqFt').val())" Onclick ="MyFunction"></asp:TextBox>