In the game directory there's a file called Benchmark.exe. When you click on that it brings up a small window where you can choose which graphic preset to run, then what graphics sub-settings, and how many times to run it. Then you click the Run button.
That said, it crashed every time I tried it before it even started a benchmark. I do see though that there seems to be a hotfix patch just released, which includes "Benchmark tool fixes" in the patch notes.
Just found this Gamers Nexus video which confirms how buggy the in-game bench tool is, at least before the hotfix. It's hard to say if the hotfix completely fixes it's quirks and inaccuracies, but since MSI Afterburner can give you more comprehensive info which you can set to show onscreen WHILE playing the game, vs a mere flyby, I tend to think the 3rd party tool bench is a better way to go.