I'm having conversions from banners on a particular site which also has popunder from Popads. I'd like to try the pops on that particular site as well but I have no idea how to get the website ID for whitelisting on Popads directly. I contacted Popads support, gave them the site URL and asked for site ID in their system but they avoid to reveal that information. Does anybody know how to get Popads website ID for a domain I already know?
Since you already know the domain you can try to monitor the HTTP headers of the pops served from that website. Also I would try to check the page source info if any publisher id is visible from there 
It's typically a 5 to 7 digits number, maybe the one from the cookie at the bottom? (Really not sure).
What geo is it?
Seems like I need to whitelist all the numbers I see there!
Just look into the source code and look for a javascript section that contains this:
_pop.push(['siteId', 123456]);
Where 123456 is your site/zone ID.
Alright, I found the site ID. Popads code was encoded and I had to decode here: http://ddecode.com/hexdecoder/ and then I can see this part in the final code: "siteId", #
Great that you got it!
Popads is still easy to reverse engineer, but some sources hide/encode their IDs in a much more difficult manner.
Especially some native networks are quite hard to backtrack.
There is a much easier way to find the placements IDs for any traffic source you want. With the help of adplexity you can do a search by publisher with the specific site you have and you will get all the ads running in that site, after that just check the ad trends and you will find the placement ids for every traffic source.
In the following example i did a search for: http://www.peliculasmega1k.com/

After that just filter only the Popads traffic and check on the ad trends the placements ids.
You can do the same process for any other traffic source.