Connect to icobench.com API in R with public and private key

Hello,
I'm used to do data science in R. However I have to connect to icobench.com API, and I don't manage to authenticate with the private and public keys they give me.

Here is their web page in which they explain what to provide in the query : https://icobench.com/developers#manage_api_access

Here is my code in R :

privatekey = "private key they gave me"
publickey = "public key they gave me"

signature <- openssl::base64_encode(openssl::sha384("", privatekey))

req = GET("https://icobench.com/api/v1/icos/ratings",
authenticate("user@gmail.com" , "password"),
add_headers("X-ICObench-Sig"=signature),
add_headers("X-ICObench-Key"=publickey))
stop_for_status(req)
content(req)

I need someone to find the right code so I can authenticate properly.