I managed to get into the WWE Thunderdome two times in the last week. The first time was at Hell In A Cell 2020 and the second time was the following SmackDown. If you ever want to take part in the WWE Thunderdome, make sure to check out WWE's Twitter or Facebook accounts for registration posts. I would recommend turning on notifications for WWE's Twitter profile because you'll be able to register faster, and speed is important here because "seats" get filled very quickly.
Once you've signed up for the WWE Thunderdome, you'll get a confirmation email usually a day before or the day of the show and you'll receive another email with a link to join the WWE Thunderdome. The email will also give you a call time to join.
Some useful tips:
1. If you join the link on your call time and the WWE Thunderdome is full, keep refreshing the page until there's a space to join.
2. When your in, make sure to be very enthusiastic on camera. The more enthusiastic you are, the more likely you'll be put on the camera side and even in the front row.
3. Don't swear. Even though your voice won't be pumped into the WWE Thunderdome, joining requires you to have your microphone on and WWE moderators are actively looking to make sure people are following the rules. If you swear, you could be cut from the WWE Thunderdome.
4. Make sure to wear WWE merch on camera as you'll more likely be shown on camera.
5. Use a laptop or computer over a mobile phone and make sure the camera is stable and not moving around.
This is what it will look like once you've entered the WWE Thunderdome: