Batch append/prepend text to the current output filename of all QUEUED render items.
Find out more about it here: AE Script: Batch Rename Render Queue Item Filenames.
Open After Effects, then navigate to File > Scripts > Run Script File... and choose RQ_Apprepend.jsx. The RQ Apprepend panel should appear.
To setup as a dockable panel:
Copy the RQ_Apprepend.jsx file into the Scripts > ScriptUI Panels folder, found in the After Effects application installation folder. Then open AE and launch it from the bottom of the Window tab in the menu bar.
To setup a scripting environment, refer to this guide: Quick Setup for After Effects Scripting.