First you need to generate flightplan in simbrief, then click "Download FMS", scroll down until you see "x-plane 11", click download then drop the file in "x-plane root folder\Output\FMS plans\" and then you can load it through company routes.
which aircraft are you flying? it really depends as they use many different formats. Default is .fms, FF757-777 uses .flp, and SSG stuff they use .ufmc....so you will need to download them accordingly.