Once you ordered your game hosting, you will receive 2 or even 3 mails. Once with your order, once with a few information about the server and the pricing. 2 Mails are not necessary, the one with the pricing and the one with the invoice if you already paid. If you didn't pay, for now, you will not receive the "important" mail.
The mail you should have received should look like this :
This mail is critical. You can't directly login via > Client Area > Your Service > Go to the panel without it.This mail is to set up your password.
Once on this page follow the instructions to reset your password. (It's not the same account as in your client area)
And you will be directly logged in :
The first time login is the hardest now you can always log in at https://game.polisystems.ch without all these steps or through your client area > (Factorio Stack Inserter for example) > Go to the panel.