Create a Superuser in postgres

I'm looking for setup a Rails Environment with Vagrant, for that purpose the box it's been provisioned through bash shell method and includes among others this line: ```sudo -u postgres createuser <superuserusername> -s with password '<superuserpassword>'``` But I'm getting a configuration error: > createuser: too many command-line arguments (first is "with") Can you help me with the correct syntax for create a Superuser with a password. Thanks.