welcome to TTD!
you should see when you post a bar on top of the post area. the icon that looks like a strip of film... click that as that is "insert video" ... paste the url of the video within the bracketed coding . on my screen, it's the 2nd row that starts with B I U ... to the right of those, you should see 2 globes, a picture frame and then the strip of film.
alternating you can manually type out the code and enclose the video embed url with the coding
[ video] url [ /video ]
no spaces though after the left brackets and no spaces before the right brackets. posted above with spaces so you know the formatting code
HTH