According to this question and one of its answers (https://answers.splunk.com/answers/4106/include-jquery-into-setup-xml.html), you may be able to put the JS directly inline via the <script>...</script> tags.
I cannot find any Splunk documentation that mentions another way to load JS files, but you may be able to use a method that is available to HTML dashboards by trying to load a JS file in your app with the following line. I have not tested it, but this is how one includes JS files in HTML dashboards.
<script src="/static/app/<YOUR_APP_NAME>/<JS_FILE_NAME>.js"></script>
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