You can post that stuff if you have a high enough “trust level” on the forum. There’s an explanation of trust levels and the criterea to meet them in this post (it’s down a bit after the formatting tips).
In the meantime, you can post links people can easily copy/paste using the preformatted text option. (It’s the </> symbol on the edit bar on desktop, and you can also get the same effect by enclosing the link in backticks (the key next to the 1 on a standard keyboard.) It’s also helpful to include such links on their own line, as that also makes it easier to quickly copy and paste.
So, typing this
`www.blizzard.com`
becomes
www.blizzard.com
It is important to note that just posting a link to your youtube/twitch can end up falling into the advertising category, which is against the forum rules. (Things like posting a video guide in response to a question are fine, as well as to add some additional info to a post. If you’re adding a video to a thread you’re starting, a good rule of thumb is that people should be able to discuss the post even if they don’t watch the video.)