code.js

const selectionNodes = figma.currentPage.selection;
const nodes =
  selectionNodes.length === 0 ? [figma.currentPage] : selectionNodes;

nodes.forEach((node) => {
  if (node.type === "INSTANCE") {
    addAnnotation(node);
  } else {
    node.findAllWithCriteria({ types: ["INSTANCE"] }).forEach(addAnnotation);
  }
});

figma.closePlugin();

function addAnnotation(instance) {
  if (instance.annotations.length) {
    console.log(instance.annotations);

    /**
     * The following deletes existing annotations
     */

    /*
    instance.annotations = [];
    */

    /**
     * The following appends to existing annotations but loses existing rich text formatting.
     * Rich text is currently unsupported in the plugin API.
     */

    const current = instance.annotations[0];
    const currentProperties = current.properties || [];
    const existingMainComponent = currentProperties.find(
      (a) => a.type === "mainComponent"
    );
    if (!existingMainComponent) {
      instance.annotations = [
        {
          labelMarkdown: current.labelMarkdown,
          properties: [{ type: "mainComponent" }].concat(currentProperties),
        },
      ];
    }
  } else {
    instance.annotations = [{ properties: [{ type: "mainComponent" }] }];
  }
}