I believe what you are looking for is the attribute “example” I haven’t tested this, but hopefully this gets you on the right track. Let me know how you go.
From this page: https://www.advancedcustomfields.com/resources/acf_register_block_type/
It mentions
$settings
(array) (Required) Array of arguments for registering a block type. Any argument from the JavaScript registerBlockType() function may also be used.
Looking at the Gutenberg Docs:
https://developer.wordpress.org/block-editor/developers/block-api/block-registration/#example-optional
So for example if you have a block with a Title and Description fields you add them into an attributes array:
acf_register_block(array(
'name' => 'testBlock',
'title' => 'Test Block',
'description' => 'A Test block which should hopefully show an example in the help panel',
'render_callback' => 'testBlockRender',
'category' => 'formatting',
'icon' => 'admin-comments',
'keywords' => array( 'Test' ),
'example' => array(
'attributes' => array(
'title' => 'Title',
'descrpition' => 'A Description',
),
),