HelloWorldExtensionApplicationCustomizer.manifest.json

{
  "$schema": "https://dev.office.com/json-schemas/spfx/client-side-extension-manifest.schema.json",

  "id": "59ecf2f4-974d-4974-a341-d83c27f0fd62",
  "alias": "HelloWorldExtensionApplicationCustomizer",
  "componentType": "Extension",
  "extensionType": "ApplicationCustomizer",

  // The "*" signifies that the version should be taken from the package.json
  "version": "*",
  "manifestVersion": 2,

  // If true, the component can only be installed on sites where Custom Script is allowed.
  // Components that allow authors to embed arbitrary script code should set this to true.
  // https://support.office.com/en-us/article/Turn-scripting-capabilities-on-or-off-1f2c515f-5d7e-448a-9fd7-835da935584f
  "requiresCustomScript": false
}