The scope mentioned by the OP does not have a 50 Ohm input impedance selection. He needs to get a termination adapter, but the best, most accurate measurement is by using a commercial attenuator with a terminator rather than a dummy load and tee or homebrew, uncalibrated, frequency dependent resistive attenuator. I have 20 and 100 Watt 20dB attenuators that I can follow with various other attenuators to get a safe level for harmonic and power analysis with a TinySA spectrum analyzer or power meter.
Gary
W9TD
Gary W9TD