Fixed is not correct because that would imply the closed end surface is completely rigid, which is not the case.
Is the closed end a flat surface or a hemisphere? I will assume it is a flat surface.
Frictionless is not correct because that will hold the closed end to the plane of the surface. That will be an improvement over Fixed because radial expansion will be permitted, but it will not allow the bowing out of the surface that the pressure will do to the end face.
Select the end face and insert a Remote Displacement, Behavior = Deformable, and set all six degrees of freedom to 0. That will allow for the free expansion of the end face with pressure while holding it in place.