In order to use this JS library, add a maven dependency to your pom:
<dependency>
<groupId>io.jenkins.plugins</groupId>
<artifactId>popper-api</artifactId>
<version>[latest version]</version>
</dependency>
Then you can use Popper.js in your jelly files using the following snippet:
<st:adjunct includes="io.jenkins.plugins.popper"/>