Pachi kindly wrote a FAQ about how the new forums work with Discourse
https://eu.forums.blizzard.com/en/wow/t/the-ultimate-forum-guide-codes-trust-levels-faq
It explains trust levels. Trust levels are gained by meeting certain levels of activity and need to be maintained. If you aren’t active enough then your trust level can drop again or if you misbehave and receive a time out from the forums then your progress resets and you have to start over.
You can see what trust level you are by going to your forum profile and putting .json at the end. For example this is mine
https://eu.forums.blizzard.com/en/wow/u/punyelf-draenor.json
At level 2 you can post links from trusted sites such as Youtube for example. If you want to post a link from another site you can simply type in the url, highlight it and use the </> button and it will appear as my first json example above.