Why An RF wattmeter samples the radio frequency energy traveling along the transmission line, so it has to be placed in series in that line, with the transmitter on one side and the antenna (or tuner/antenna) on the other. Most such meters are directional couplers that read both forward power going toward the antenna and reflected power coming back, which is also how an SWR reading is derived. Insert it with short, good-quality jumpers so it sees the same 50 ohm path as the rest of the feed line.
Watch out Measuring at the power supply output or in the DC cable tells you about DC current and voltage draw, not the RF watts actually leaving the transmitter.
RF power lives in the coax: put the meter between rig and antenna, not in the DC line.