EVP_PKEY_meth_set_sign
OSSL_DEPRECATEDIN_3_0 void EVP_PKEY_meth_set_sign(EVP_PKEY_METHOD *pmeth, int (*sign_init)(EVP_PKEY_CTX *ctx),
EVP_PKEY_meth_set_sign(custom_pmeth, NULL, custom_pmeth_sign);