This can be accomplished with the ch_admin_passwd binary.
[[email protected] root]# cd /usr/local/psa/admin/bin/
[[email protected] bin]# ./ch_admin_passwd --help
./ch_admin_passwd: Utility to set Plesk adminstrator’s password
Gets password from the environment variable PSA_PASSWORD
Password should be from 5 to 16 symbols and should not contain login name,
whitespace, quotes or national characters
display this help and exit
We must export the variable before running the binary. We will unset the variable when we are done.