Stop by Wally World and pick up a box of blue DEF. The injection rate is approximately 3% calculated as follows; Diesel Exhaust Fluid - DEF - DEF Usage Calculator
Best way I have found to fill the underhood tank, before the DEF add dash indicator lights up, is to pour slowly listening for the sound change as it gets into the neck of the reservoir. Always be careful to not spill or overfill, as the def has urea in it, and is corrosive.
I say that because, there has been some issues with the MB emmisions and ECM computers talking to each other, due to suspected but not admitted to, programming glitches. Just sayin'