You need the dip tube so that the poppet works properly. If the keg arrived without one, get the vendor/store to provide you with a GAS dip tube. While some (rather few) like to push CO2 in through the liquid tube, the vast majority just use the normal gas dip tube. As an alternative, you could install a gas post in the lid, and connect an air stone to that, which goes into the beer. The tiny CO2 bubbles it releases will do a good job of carbonating (if you're in a rush). Otherwise I'd just use the 2 week set and forget method (at serving pressure and temperature for 2+ weeks).