For links, highlight any text you want to be linked by dragging your mouth and highlighting the selection you'd like and using the "link" tool in the message editor, it looks like a globe with an infinity sign next to it.
Or, you can use BB code, like this
[url =
https://www.youtube.com/] [Your text here] [/url]
Without spaces, it will be like
[Your text here]
Same thing goes for images, only instead of [your text here] you will replace it with the image you want
[ url =
https://www.youtube.com/watch?v=AeWTVwSa7m8 ] [ img][Your image here] [/img] [/url]
And you get this
You click the picture to get to wherever your URL directs you.
Hope this helps