Storing referring document value as a field in the document it refers to

import {defineField, defineType} from 'sanity'

export default defineType({
  name: 'testimonialBlock',
  title: 'Testimonial Block',
  type: 'document',
  fields: [
    defineField({
      title: 'Block Nickname',
      name: 'nickname',
      type: 'string',
    }),
    defineField({
      title: 'Block Location',
      name: 'blockLocation',
      type: 'reference',
      to: [{type: 'page'}],
      options: {
        disableNew: true,
      },
      initialValue: async (value, { getClient }) => {
        const client = getClient({ apiVersion: '2023-01-18' });
  
        const _id = await client.fetch(`*[_type == 'yourType'][0]._id`)
        return {
          //return an object with the values you need 
          _type: 'reference',
          _ref: _id   
        };
    },
    }),
    defineField({
      title: 'Quote',
      name: 'textBlocks',
      type: 'array',
      of: [{type: 'block'}],
    }),
  ],
  preview: {
    select: {
      nickname: 'nickname',
      location: 'blockLocation.title',
    },
    prepare(selection) {
      const {nickname, location} = selection;

      let title = nickname;
      let blockType = 'Testimonial';

      if (location) {
        title = location + ': ' + nickname;
      }

      return {
        title: title,
        subtitle: blockType
      }
    }
  }
})