Hi…,
Welcome.
I suggest setting up SimBrief Downloader to place the SimBrief created rte files in the correct directory for the PMDG to access. Please see FAQ - Exporting SimBrief flight plans to PMDG 737 for MSFS - #3. There is an excellent option for Simbrief to automatically download the flightplan when you Generate OFP.
Create your flight plan in SimBrief, press Generate Flight to produce OFP. Download flightplan using SimBrief Downloader manually or automatically.
Load the flightplan in 737 FMS using RTE - FLT PLAN REQUEST
See also P118 of …your MSFS\LocalCache\Packages\Community\pmdg-aircraft-737\Documentation\PMDG_737_MSFS_Introduction.pdf
Please let me know how you get on.
Cheers
Ian