Can I add custom JS to Sprout Form?
You can do this with the default Email Dropdown field (no JS needed!). The Email Dropdown field lets you create a mapping between an item in the dropdown and an email:

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The Email Dropdown field obscures those emails in your template. And when you set up your Notification Email, you can reference your Email Dropdown field by its handle in your Recipients setting: {emailDropdownFieldHandle}

In the case you ever did wish to add JS to your templates, the best way to add it to your template is to use Template Overrides.