Receiving a social security number despite having a EHIC, in order to register as a artist auteur?

Hello,

I came to France to study from Germany, and since I already had my EHIC from Germany, I didn't need to apply for social security coverage here. However, I want to declare my status as an artist-auteur (same declaration form as for micro-entrepreneur), and I cannot do this without a social security number. How can I apply for a french social security number despite already having coverage from another European country?

Thank you in advance.