It depends on the policy of the journal. However, uploading of a paper or part of it in any public site, usually, considering publication, but you should inform the editor in chief of both sides about this (the first to which you uploaded and the second to which you want to send. 

Many journals ask about article submitted to another journal, presented in a conference, as said by Gupta may pose problem during plagiarism checking

Thanks all for the suggestions. I thought uploading unpublished article in SSRN will not create any problem as they do not ask for copyright. But as you said, I will withdraw my paper untill it is published somewhere.

I would recommend against it. Once your paper is published, only then upload it to SSRN.