Hi, the 737 max is planned to be added eventually. Once some better data becomes available for it.
For now I recommend creating your own custom airframe in the My Fleet section, based on the standard 737, and simply match the weights (MTOW, MLW, OEW, etc) to the add-on you’re using for best results. If the 737 max for X-Plane has accurate fuel burn (as far as I have heard it does not, but I don’t know for sure since I haven’t tried it myself), you can also try a Fuel Factor of M15 (which will decrease SimBrief’s fuel burn by 15%).
Best regards,