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For example , this is full url video from youtube :
//www.youtube.com/watch?v=6Grhs3-EuGU&feature=feedrec_grec_index

Now , if you want to post video here on forum , you have to take just one peace of this url
before  v=   6Grhs3-EuGU   & and after
So correct code is : 6Grhs3-EuGU