RPC plugin allows to register users via config file, where user name, password in hash format and permission groups are required. To make it easier, this utility can help with password hashing.
password-hasher <password> <cost>
Put desired password as first parameter, and cost value as second. Keep in mind that the cost value should match the one in security plugin, otherwise the password cannot be verified. Cost value range is 4 - 31, high numbers require a lot of memory and CPU time to process.