MB for Yoast not counting words

Hello again!

I found a tutorial on YOAST page that made the counter work for me:

/* global YoastSEO */

class MyCustomDataPlugin {
    constructor() {
       // Ensure YoastSEO.js is present and can access the necessary features.
       if ( typeof YoastSEO === "undefined" || typeof YoastSEO.analysis === "undefined" || typeof YoastSEO.analysis.worker === "undefined" ) {
            return;
        }

        YoastSEO.app.registerPlugin( "MyCustomDataPlugin", { status: "ready" } );
        
        this.registerModifications();
    }

    /**
     * Registers the addContent modification.
     *
     * @returns {void}
     */
    registerModifications() {
        const callback = this.addContent.bind( this );

        // Ensure that the additional data is being seen as a modification to the content.
        YoastSEO.app.registerModification( "content", callback, "MyCustomDataPlugin", 10 );
    }

    /**
     * Adds to the content to be analyzed by the analyzer.
     *
     * @param {string} data The current data string.
     *
     * @returns {string} The data string parameter with the added content.
     */
    addContent( data ) {
        data += "Hello, I'm some additional data!";

        return data ;
    }
}
/**
 * Adds eventlistener to load the plugin.
 */
if ( typeof YoastSEO !== "undefined" && typeof YoastSEO.app !== "undefined" ) {
  new MyCustomDataPlugin();
} else {
  jQuery( window ).on(
    "YoastSEO:ready",
    function() {
      new MyCustomDataPlugin();
    }
  );
}

But I have no idea what should I put there on the line — addContent(data).
The example shows a hardcoded text. How can I put there a METABOX FIELD ID, for example?

Any suggestions?

Thanks!