function change() {
document.getElementById('<%=btnLoadBenefits.ClientID %>').click();
}
<asp:Button ID="btnLoadBenefits" runat="server" OnClick="btnLoadBenefits_Click" style="display:none;"/>
<asp:TextBox ID="txtCompanyname" runat="server" Width="220px" Height="18px" MaxLength="50" onchange="change();">
</asp:TextBox>