If you want to put some value inside _EVENTARGUMENT you should do this with javascript before sending form by _doPostBack('UpdatePanel1','') because __EVENTTARGET is hidden field and in your html document it looks like this:

<input type=hidden value= id=__EVENTARGUMENT name=__EVENTARGUMENT>

I recommend you to do something like this:

function setArgAndPostBack() {

    var arg = document.getElementById('__EVENTARGUMENT');

    var arg = document.getElementById(__EVENTARGUMENT);

    arg.value = 'something you want to put to server';

    __doPostBack('UpdatePanel1', '');

}

If you use jQuery it would be shorter:

function setArgAndPostBack() {

    $(#__EVENTARGUMENT).val('something you want to put to server');

    __doPostBack('UpdatePanel1', '');

}

If it doesn't work I would like to suggest you to put one hidden field inside Update panel:

<asp:UpdatePanel runat=server ID=UpdatePanel1>

    <ContentTemplate>

        <asp:HiddenField ID=hdnData value= runat=server />

        <!-- your content goes here -->

    </ContentTemplate>

</asp:UpdatePanel>

And then do the same work like above:

function setArgAndPostBack() {

    //Here hidden field is filled with your data

    $(#<%=hdnData.ClientID%>).val('something you want to put to server');

    __doPostBack('UpdatePanel1', '');

}

In first scenario you are able to get __EVENTARGUMENT in server side:

String args = Request[__EVENTARGUMENT];

If first scenario doesn't work you can use something like that:

String args = hdnData.Value;//This works even in Page_Load function.