Can't connect projector to the main monitor (ASAP)

How to connect projector to main monitor (BENQ) so the projector can detect and share the monitor?

You don’t. Unless your monitor has video output. Most monitors I know has only video input. In that case you have to connect it to the computer and somewhere in settings->display you set screen duplication.

wait, sorry. I mean connect projector from main monitor to projector