There are many ways to add a custom JS files to Service Portal. If it's specific to one or more widgets, you can use "dependencies" or "angular providers"... or if you just want to include it on page load you can add it to the Theme using the "JS Includes" related list.
Thanks for your update. We have another issue like we have to reference field on portal. On basis of first field we need to populate the value in second field.
