Hi Kerem, you can find this in C:\Program Files\Windows SteadyState\XML\Restrict.xml:
<restriction id="chkRestrictIEToolbarExtensionButton" name="Third party extension buttons" _locID="chkRestrictIEToolbarExtensionButton" high="on" medium="off" low="off">
<poke resid="chkRestrictIEToolbarExtensionButton" regkey="Software\Microsoft\Windows\CurrentVersion\Policies\Ext\RestrictToList" type="REG_DWORD" on="1" off="0" delete=""></poke>
Hope this helps!