Generate CSR using Mbed TLS and Public Key

Hi

I am trying to generate an CSR in my embedded system. On my system, there is a crypto device where private key is stored, and i cannot access it outside of the crypto device but public key is available. Example application “cert_req” available with mbedtls expects private key as the one of the input parameter to the application. My requirement is to use the public key instead of private key in the available “cert_req” application to generate the CSR. Can anyone please guide me for the same?

regards
Noufal