Yeah, not sure where it is – or even if it was exposed back then (or even implemented at all in 4.5). I started at Epic at 4.7 so 4.5 is even before my time. I don’t currently have time to download a new 4.5 binary from our launcher to check it out but looking at that screenshot, it’s probably in one of the other tabs (General Settings), etc. You should be able to search in the top for “streaming” probably in the search bar, though maybe that feature was added after 4.5.
The unfortunate thing though, is even if you did enable it, streaming wasn’t fully supported until 4.16 and was barely functional on PC before that.