You can call custom activities from data transform using callActivity() function.

Pass primaryPage and activityName to callActivity.

Syntax will be-

pega_rules_utilities.callActivity(PrimaryPageName, ActivityName, tools.getParameterPage());

Create an activity called NewDefaults activity in your Class and ruleset and call the activity from this activity. This activity will run automatically no trigger point. 

You can call the activity use the OOTB call activity function also as mentioned in the above post

There are two appraoches:

1. Write a javascript function and include this JS in portal harness script and styles tab. Attached pega onload function and call above function as callback. This is working fine in production in many implementaitons.

Look for pega doc for exact syntax of ~~pega.u.d.onload()..

2. Not suggetable: But works from sure. In the operator record, you will have Data-Portal.ShowDesktop activity, you can create your own activity and write the logic you want to achive and call OOTB activity in the last step. This requires updates to each and every operator record.

NewDefaults will be executed only if NewHarness is rendering. In this case, New harness will not be loaded.