Machan adding the whole url of the video will not work. If you take this link there is a part 'watch?v=LdOQdu6Io8k'. Now after the '=' sign you have the video's unique code (LdOQdu6Io8k). That part ends with '&' sign. After '&' it describes something else. Only add that part between the YouTube tags
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[tags]LdOQdu6Io8k[/tags]