If you just want to cover instead of parging - each ICF has a screw strip - typically 8 " on center and they will start about 2" down from the top. They are embedded 1/4 to 1/2" inside the foam so you can affix drywall (required for interior walls fire proofing). If you just wish to cover, then cut your 1/2" pressure treated ply wood to the correct height and screw it to the foam with coated deck screws into the plastic screw tabs. Be sure to only screw in until it is tight and do not over torque or the plastic will yield and the screw will spin. As
@Bofobo says you can then add a topper of 2 x to make a ledge for sitting or putting things on.