It is a value on the Pathfinder by Roll20 sheet, but I have not noticed it any documentation. Presumably, an entry like "+# [bonus type] to @{critconfirm_bonus}" would work, but I have not tested it. No, after testing every variation ("crit confirm", "crit confirm bonus", "critical confirmation", @{critconfirm_bonus}", etc.) I found none that actually did anything; the "[CRIT CONFIRM BONUS]" value in the die rolls always showed +0.