Actually, you will need to use the option "Manage Clients" on your official Tibia Client, and select the Test Server Version. If you are invited to join it, you can log on the test website in order to post topics and contribute to the test server discussion, but it won't work if you attempt to install a new client from there. The permission to join it depends on each occasion, and you will be notified about being invited earlier. Usually by the end of the Test Server period, everyone is granted a chance to play it.
Just go to the option of "Manage Clients" on your regular Tibia Client and switch to the Test Server one. After the download of updates, you will be able to play it normally if you were previously invited to it.
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