Is it possible to establish a TLS1.3-session without any certificates/authentication?
It is possible to do the authentication without certificates but it is not possible to do no authentication at all. By using PSK the server authentication is not done with a certificate but with a pre-shared key, i.e. a common secret known between client and server.
If yes, how can I instruct an openssl client (via commandline) to do so?
With openssl1.1.1 you can start a server the following way:
# use a 48 byte PSK
$ openssl s_server -psk $PSK -cipher TLS13-AES-256-GCM-SHA384 -nocert -accept 2020
If you then start the client with the same PSK you get a successful TLS 1.3 handshake with no certificates involved:
$ openssl s_client -psk $PSK -connect localhost:2020
no peer certificate available
No client certificate CA names sent
Server Temp Key: X25519, 253 bits
SSL handshake has read 195 bytes and written 475 bytes
Reused, TLSv1.3, Cipher is TLS13-AES-256-GCM-SHA384