Hi,
Please try to use this code to register the fields and preview area for a custom block
'preview' => [
'image' => 2140, //image ID
'name' => 'William Shakespeare',
'content' => 'Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam'
],
'fields' => [
[
'type' => 'text',
'id' => 'name',
'name' => 'Name',
],
[
'type' => 'textarea',
'id' => 'content',
'name' => 'Intro',
],
[
'type' => 'single_image',
'id' => 'image',
'name' => 'Image'
],
],
Remember to create the property name as the same as the field ID and the preview area will render the template of the block with the preview value.
Screenshot https://imgur.com/RXAihQ4