How to create a reference mark in Sanity using block-tools and a portable text schema.

export default {
  name: "hjelpeartikkelLink",
  type: "object",
  title: "Lenke til en hjelpeartikkel",
  fields: [
    {
      name: "reference",
      type: "reference",
      to: [{ type: "hjelpeartikkel" }],
      validation: (Rule) => Rule.required().error("...")
    }
  ]
};

{
  name: 'content',
  type: 'array',
  of: [{
    type: 'block',
    marks: {
      annotations: [
        {
          name: 'linkInternal',
          type: 'object',
          fields: [{
            name: 'target',
            type: 'reference',
            to: [{ type: 'page' }]
          }]
        }
      ]
    }
  }]
}

/* other stuff */
return {
  type: '__annotation',
  markDef: {
    // extract properties from <a /> and
    // get the sanity document id
    _type: 'linkInternal',
    _key: randomKey(12),
    target: {
      _type: 'reference',
      _ref: relevantDocumentId
    },
  },
  children: next(el.childNodes)
}

export const link = (url, children) => {
  const key = randomKey(12);
  return ({
    _type: "__annotation",
    markDef: {
      _type: "link",
      _key: key,
      href: url
    },
    children: children?.map(it => ({ ...it, marks: [...(it?.marks || []), key] }))
  });
}

if (tagName === "a") {
  return link(el.href, next(el?.childNodes));
}