Method-1 (Computer)

1. Click right button of mouse from video
2. Click "Copy Video URL"
3. Paste into input box

Method-2 (Mobile)

1. Go to youtube video
2. Click Three Dot button from right side of video
3. Click "Share" option from dropdown
4. Click "Copy link" button
5. Now paste into input box

Click on Three Dot button of your post

From dropdown click "Embed" option

Click "Copy Code" button for copy and paste into input box