I wonder how to configure IMAPSieve with user scripts. I can't find much
information on the internet.

try to enable managesieve:

service managesieve-login {
  inet_listener sieve {
    port = 4190
  }  
}


https://wiki2.dovecot.org/Pigeonhole/ManageSieve/Configuration

Roundcube or Thunderbird with Sieve plugin connect to your dovecot server on port 4190.
Users are authenticatd with the same credentials used for pop/imap and can upload their
scripts.

Best regards
Gerald