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There is no exact phrasing that is needed. You just need to let it be known that you are refusing to answer based on your 5th amendment right against self incrimination.
If you want some exact phrasing though, you can say: " I am electing to raise my 5th Amendment right against self incrimination. I am not going to answer, on the grounds that any answer I give, could incriminate me."
That will work just fine. You will still have to answer preliminary questions though. You can refuse though, anytime your answer involves you participating in a potentially criminal act. Good luck