SF_15.1 - https://www.progress.com/documentation/sitefinity-cms/next.js-renderer-widgets

CustomWidgetRegistry.ts

import { WidgetRegistry, initRegistry, defaultWidgetRegistry, CSRFormComponents } from '@progress/sitefinity-nextjs-sdk';

const customWidgetRegistry: WidgetRegistry = {
    widgets: {
...
}
};

customWidgetRegistry.widgets = {
    ...defaultWidgetRegistry.widgets,
    ...customWidgetRegistry.widgets,
    ...CSRFormComponents
};

export const widgetRegistry: WidgetRegistry = initRegistry(customWidgetRegistry);