During Legion, if I remember correctly, there was a button to run the simulator using the setup generated by BiB (or BiS). This was very helpful to double check / tweak the suggestion before exporting it to the WoW Addon. Was this feature removed or is it somewhere else now?
It was removed, because many users were assuming they needed to do exactly what you just said, while the site actually does the relevant computation in the background already.
The closest thing to do is equip your BiB, then grab the addon export from the game and use that to import into the Simulator.
Yeah, that feature ended up being misleading to people… we’re going to work on adding some sort of convenience to the simulator so that people can simulate their BiB and BiS setups easily.