Populate custom field with last 4 of SSN

[aschaff36]

I am a brand new administrator with limited resources and java script experience. I am attempting to create a field interaction whereby I take the value from the Candidate field 'ssn' and I want to take the last 4 numbers and populate a custom text field with those values.

Does anyone have a field interaction script they wouldn't mind sharing? Here is my attempt which is not working.

// Install on customInt10 for Candidate
// Run on Init and On Change
let activeValue = API.getActiveValue();
let customInt10 = API.form.controls['ssn'].value
if (ssn === '') {API.setValue('ssn', customInt10);
}

Thank you in advance!