You cannot set password for users internally inside IBM Worklight Application Center, that is, there is no Application Center UI for it. You must set it by configuring the web server. The web server authorizes who can access the web application, but Application Center (as web application) does not manipulate the authorization mechanism of the web server itself. Application Center is no web server administration tool (and that's by design, for web server security reasons). Hence it assumes the user list and passwords is handled externally to Application Center.
For instance, in Tomcat, you edit tomcat-users.xml. In Websphere Application Server, including Liberty profile, there are several mechanisms, for instance edit server.xml. If you use LDAP, you must add the user to your LDAP. All this happens outside Application Center.