The offset parameter needs an array. But since it's just a parameter, you can actually use .SetEffectParamater:
renderer.SetEffectParameter(FF_DISPLACEMAP, PP_DISPLACEMAP_OFFSET, [obj_camera.x, obj_camera.y]);
After uncommenting the PP_DISPLACEMAP_OFFSET code at the location you specified, I observed that it was also affected by the camera. Upon closer inspection, the PP_SINE_WAVE_OFFSET effect is actually camera-dependent as well. The shaking amplitude intensifies when obj_player moves. Please check if this matches the phenomenon I described? If so, does this meet your expectations?