...mobj it creates, so:
local spawnedparticle = P_SpawnMobj(p.mo.x, p.mo.y, p.mo.z, MT_CUSTOMPARTICLE)
spawnedparticle.momx = P_ReturnThrustX(p.mo, p.mo.angle, -4*FRACUNIT)
spawnedparticle.momy = P_ReturnThrustY(p.mo, p.mo.angle, -4*FRACUNIT)
This spawns a particle and makes it move backwards...