RF Power measurements with Spectrum Analyzer

I'm trying to learn how to properly measure output power with my spectrum analyzer. I have a transmitter datasheet that claims 15dBM typical output power with BPSK. Assuming the matching network is correctly matched to 50 Ohms, and I can configure it for constant transmit mode. Then if I just plug this transmitter into my SA should I expect to see a peak of 15dBM (well minus any cable loss) or should I expect to see half that because the source impedance is 50 Ohms so it acts like a voltage divider.

Or have I missed the mark entirely and I need to integrate the power over the entire channel width or something like that.

Data sheet is not very helpful it just says what I said above.