When trying to set a custom Transform for use in an Account Create Profile, it isn't possible to select it from the drop-down in the Create Account UI (even though the documentation here says that it is possible: https://community.sailpoint.com/t5/IdentityNow-Articles/Building-Transforms-in-IdentityNow/ta-p/7339... ). It must be configured by using the REST API as described here: https://community.sailpoint.com/t5/IdentityNow-Wiki/How-to-Edit-the-Source-Create-Account-Profile-vi... .
The Account Create UI will only allow you to configure: