You can't until you have ten post and maybe a full day. But you can write the URL to get to your video or pic's if you just simply write the URL in the plain text of your message. Remove the https:// and put a space on either side of the . then it's no longer a URL and the SPAM filters will let it pass.

Some hosting sites require you to be a member to view or get overly concerned about just anyone viewing, so you might try to access that URL without logging in to see if anyone else can too. Some sites have a special URL you have to get through their "share" function.