Hi @arunshankar
Vanilla JS is supported by default. If you would like to write jQuery then currently powerapps portal is using JQuery 3.0.
More details are here.
Also, there are many places where you can add JS / jQuery
In portal management:
For Basic form
1. Click the basic form > Additional Settings> You can see "Custom JavaScript" at the bottom.
For Entity lists
1. Content> Lists > Select the list > Options > Custom JavaScript
For Page,
Content > Web Pags> Select your desired page > Localized content > Advanced > Custom Javascript
Hope it helps.
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