Hi,
I am in OPT and working as full time now. I know I don't have to pay FICA and FICA MedC taxes, but my employer/company will not stop withholding those taxes for me UNLESS I have a written certification to prove that I don't have to pay the society security taxes." So I just want to know how I can get this kind of certificate showing that I can exempt from FICA taxes. Please let me know. Thanks a lot!

Contact the Social Security Administration.

I doubt you will get a certificate, but after you explain your circumstances, they should provide you with at least an email.