Use the amplified Zone 2 output on your AVR, which we have to assume you have since you didn't tell us what you own, and connect it to the 'A' input on the speaker selector.
Z2 Left + to the A input, Left + connection
Z2 Left - to the A input, Left - connection
Z2 Right + to the A input, Right + connection
Z2 Right - to the A input, Right - connection
Done.
If your AV receiver doesn't have a powered zone 2 output, or any zone 2 output, then you can't really do this at all. If your entire usage for the AV receiver is to power these speakers, only, then you will use the main left and right outputs and you need to setup the receiver for stereo only audio output. You also will want to ensure the volume is at a specific level.
Not sure how you are controlling volume or selecting sources or what sources you are using... or much else. So this is all just following 'standard' connectivity of a speaker selector to a normal AV receiver.