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 "Share" option
3. Click "Copy link" button
4. 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

Click on Three Dot button of the post

From dropdown click "Embed" option

Click any of the option "Embeded Video" or "Embeded Post"

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