A method for forming a headrest, which involves pre-assembling an incomplete headrest unit including a trim cover assembly having an opening therein and a headrest frame having been placed via the opening within the trim cover assembly, and a foaming process for subjecting such incomplete headrest unit to a foaming by injecting a liquid base foaming material into the interior of trim cover assembly and curing that foaming material, so as to produce a final headrest. In the method, an injection nozzle device is utilized, which has an entrapping or catching element provided at the nozzle portion thereof. Prior to the foaming process, the nozzle portion of injection nozzle device is inserted via the opening into the interior of trim cover assembly in a first direction, and then drawn in a second direction opposite to the first direction, thereby causing the opening to be closed positively by the catching element, to prevent leakage of the liquid base foaming material from the opening. Further, at least one extension may be formed at the opening, which extends continuously from the trim cover assembly into the interior of the latter, so that such at least one extension may be caught by the catching element for the purpose of positively closing the opening against the leakage of liquid base foaming material therethrough.