Not everyone may know this, and it may not be obvious. Once you have a .Win account on one sub, you can use the same account on all .Win subs.
If you have an account on one .Win sub, just log-in with the same log-in info on another .Win sub. You don't have to make an account more than once.
I know it is weird that (as far I know) you are not automatically logged-in on other .Win subs when logged-in on one of them. In a sense they are basically websites of their own while also being able to share accounts some how.
EDIT: Ofcourse you should make sure you only do this with Official .Win subs, which you can find a list of here.
It's the difference between what your browser sees and what the server's doing on the back end. Because the .win top-level domain does not uniquely identify the .win network, there's no way for your browser to know that it can send your kotakuinaction2.win cookie to thedonald.win but not to example.win. But on the back end, kotakuinaction2.win and thedonald.win might both point to the same server that accesses the same authentication database.
Which is frankly not a good thing. kotakuinaction2.thedonald.win might be unwieldy, but it ensures that somebody using the wrong subdomain (kotakuinaction3.thedonald.win) will just get an error from the server, while kotakuinaction3.win could potentially point to an attack site completely independent of the .win network.