Pucket Brothers t650 adding hydraulic

I bought a pucket brothers paver runs great tbh had 90 percent of the hoses replaced I have never ran a paver before in my life always subbed it out but ive done a few jobs with it with great success, how ever the hydraulic pump sheared the shaft, I had the pump rebuilt but now im curious how do I add hydraulic oil to the machine? it has a small reservoir with a cap on it and a warning label not to remove it. ill post some pictures of it later if that helps