This UI without a doubt is a pain in the backside, it isn't in the least intuitive, but currently there is a sort of short cut to get there. AvastUI > Menu > Settings > General - Troubleshooting - scroll down to the bottom and select Open old settings. From that screen select Components > Web Shield > Customise > Exclusions.
I hope this helps, but this may not be there indefinitely as this UI, settles on what the hell will be in it.
A real pain, not to mention some of the settings (in the mainUI not the old settings mentioned above) are actually hidden in what is called the geek:area but there is nowhere in the UI that even mentions this.
So how the hell do you get into the geek:area, having got into the Menu > Settings click the search button and type geek:area (Avast Geek > Secret tech cave of advanced settings for skilled Avast users). Totally easy if you know about it, but who the hell does.