Some post require code review which were posted in Stackoverflow , such posts have to migrate to https://codereview.stackexchange.com/ . There should be option in flagging, when selected "it dosenot belongs here", under this i feel codereview.stackexchange also be available.
This is not going to happen. There are a maximum of 5 unlocked migration paths allowed per-site, and Stack Overflow already has its 5 filled. You would need to make the case that one of the existing paths should be removed and replaced with Code Review.
In addition, the Code Review site is still in beta, and migration paths will definitely not be unlocked for sites that are still in beta. Even if you made the case that Code Review should replace one of the existing migration paths, that won't happen until after Code Review comes out of beta.
However, there is a workaround. If you think a question desperately needs to be migrated to a site that is not unlocked in the list of migration paths, you should flag it for ♦ moderator attention. Moderators can migrate a question to any site in the Stack Exchange network. When flagging, choose the "other" option so that you can explain which site you think the question should be migrated to along with a short explanation of why you think it should be migrated there.
Alternatively, simply vote to close the question as "off-topic" without specifying a particular migration destination, and leave the asker a comment explaining the existence of the Code Review site and why you think their question would be better asked there. This is generally what I do for sites like Ask Ubuntu, Code Review, Unix/Linux, and IT Security.