You can configure it using the console: https://docs.oracle.com/middleware/12212/wls/WLACH/taskhelp/security/ConfigureCustomUserNameMappers.html

I cannot use the console due to various reasons. How do you do the same thing in the Weblogic Scripting tool?

I already have example code for a Custom Username mapper, I need to figure out how to tell weblogic about it.