Copy url of the site, like a you tube page, go to Insert tab at...

Copy url of the site, like a you tube page, go to Insert tab at top of reply box, insert as media, another box will pop up and you just paste the url into that, hit insert, then hit post message and it will show when posted.
I'm still using old HC but the new one is similar.

Gif sites like:

https://giphy.com/

allow you click on a gif and then to copy a media url and insert as an image. like this


Have a go.