OPT Extension working at a Client Site

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Hi,

I am working at a Client Site through Collabera, need to apply for my OPT Extension in couple of weeks. My employer is Collabera, Will there be any issues getting my Stem Extension approved as there has to be a proper Employer- Employee relationship etc., How do I make sure I am submitting required documents and I -83 write up?

Thanks in advance for your responses.