Setting default slug and making it readOnly in Sanity.io

export default {
  name: 'blog',
  title: 'Blog',
  type: 'document',
  initialValue: {
    secondaryTitle: true,
  },
  fields: [
    { title: 'SEO', name: 'seo', type: 'pageMeta' },
    { title: 'Page Header', name: 'simpleHero', type: 'simpleHero' },
    {
      name: 'slug',
      title: 'Slug',
      type: 'slug',
    },
    {
      name: 'secondaryTitle',
      type: 'boolean',
      title: 'Secondary Title',
    },
    {
      title: 'Featured Article',
      name: 'featured',
      type: 'reference',
      to: [{ type: 'post' }],
    },
    {
      title: 'Featured Categories',
      name: 'categories',
      type: 'array',
      of: [{ type: 'reference', to: [{ type: 'category' }] }],
      validation: (Rule) => Rule.required(),
    },
  ],
  preview: {
    select: {
      title: 'simpleHero.title',
    },
  },
};

initialValue: () => ({
    slug: {_type: "slug", current: `${randomstring.generate({length: 12, charset: 'alphabetic'})}-${Date.now().toString(16).toUpperCase()}`}
  })