Edit: Adding a quick minitutorial on embedding a youtube video, and Cheeseman has a slightly different method below, just click the Spoiler button.
[ Spoiler ]
It's a nice Friday today (...
You can still place a video even if you’re using your phone. These steps if you’re on an iphone using safari. No idea for android.
Step 1: Press the “Aa” button beside the address bar below then select request desktop website because if you’re on the mobile version, you can find the icon on step 2. You can only do this by pressing reply not the quick reply.
Step 2: The video icon will now pop out.
Step 3: Select “From the web” and paste the youtube link there. You can then select the size of the window/video and then insert then post your reply.