manifest.json

{
  "name": "hello_plugin",
  "id": "896250432212712949",
  "api": "1.0.0",
  "main": "code.js",
  "ui": "ui.html"
}

ui.html

<div>
    <h3>Hello Figma Plugin</h3>
</div>

code.js

figma.showUI(__html__)