Install grounding cable with eyelet to outlet, no center screw
The round &quot;D?&quot; outlets use 1 centre screw. The square outlets use 2 screws for the cover.
Both are grounded.
You can file a 1mm slot for the wire exit and fasten to the metal box outlet then cover or use an external centre wire with a ringlug that is a threadtight fit.  Your choice.
You can buy or make these and somewhere there is a 1Mohm R in series.
Removing the cover should reveal the bare copper ground wire fastened to the box. Just fasten any suitable wire to the box, file a relief slot for the wire to exit on the edge of the drywall and cover then connect to matt and use strain relief.
Voila!