A seat for a vehicle includes a seatback support structure including a main portion having a first side and a second side disposed opposite the first side, and a flange that projects from the main portion in a direction away from the first side, and that defines a hole extending through the flange for receiving a headrest post. The seat further includes a trim cover disposed over at least a portion of the seatback support structure and a trim fastener attached to the trim cover for securing the trim cover to the flange of the seatback support structure. The trim fastener has two opposing side portions and a cavity therebetween for receiving the flange.