From within Pharo:

| rsa |
rsa := LcRSA generateKey: 2048.
LcX509Request new
  setSubject: 'www.samadhiweb.com';
  setPublicKey: (LcEvpPublicKey setRSA: rsa);
  sign;
  asString

The output is an X.509 certificate request, suitable for Let's Encrypt: