Above the text box (where you type your message) there is a row of icons. The last one on the right hand side is the quote wrap, and to the left right beside that, is the Insert Video icon. It will say Insert Video when you hover your cursor over it. Click it. An app opens, telling you to Enter your video clip URL below. Once you do that, say okay, and it should be good to go, although, some people have problems doing this, and it may not work on certain devices (I use an HP Pavilion laptop ) It shows up as text until you post it, but you can always check by Previewing your Post
[video=youtube;T1D3ejwQiVg]https://www.youtube.com/watch?v=T1D3ejwQiVg&list=PL670A24D3E55556DB[/video]