Thanks for reply Carl.

So if I, by trial and error, install HS210 on the "power" side instead of the "load" side, then I can operate the lights with the manual switch as well, correct? I won't have to leave the manual switch in "On" position for the HS210 to work properly, right?

Thanks.

A few details you can use in this situation, not to be attempted by those without some experience with electrical work.

You can determine the line from the load 3 way switch with a meter a few moments of work.

Make sure the light controlled by the switches is currently off.

Pull the switch out of the box

Use a meter to read the voltage between the common screw and the neutral (white) or ground wire. If it reads 120V (us) or 220V (other places) you are on the line side.  If it reads 0 you are on the load side.

The line side being the "master" and the one you will need for power to the switch to make the wifi work properly regardless of the other switch position.

Now onto the other thing you can do which again, should not be done without some expereince.

You need a neutral because the switch always need power to maintain WiFi.  In the US (don't know about other places) most homes are what is know as neutral/ground.  What it means is the neutral bus in your breaker box is attached to the box, just like the ground bus, making them effectively equal. Now all the electricians in the audience will have a fit, but it is true.  Some situations you will have an isolated ground circuit, and the outlets should be orange, hospitals computers centers etc often have it.

So why is that important?  You can put your white wire and your green wire from the switch on the same bare copper ground wire in the box and Voila! It will work.

Again, don't hurt yourself, if this does not make sense ask an electrician.