dynamic-content-custom-field-example.php

<?php
// Make sure you replace the following in the example:
// - META_KEY_HERE
// - LABEL_HERE
// - VISUAL_GROUP_LABEL_HERE

// Register your new Dynamic Content fields:
add_filter( 'et_builder_custom_dynamic_content_fields', function ( $custom_fields, $post_id, $raw_custom_fields ) {
	$custom_fields['custom_meta_META_KEY_HERE'] = array(
		'label'    => esc_html( 'LABEL_HERE' ),
		'type'     => 'any', // 'text', 'image', 'url' or 'any'
		'fields'   => array(
			// You can skip these if they are not needed:
			'before' => array(
				'label'   => esc_html__( 'Before', 'et_builder' ),
				'type'    => 'text',
				'default' => '',
				'show_on' => 'text',
			),
			'after'  => array(
				'label'   => esc_html__( 'After', 'et_builder' ),
				'type'    => 'text',
				'default' => '',
				'show_on' => 'text',
			),
		),
		'meta_key' => 'META_KEY_HERE',
		'custom'   => true,
		'group'    => 'VISUAL_GROUP_LABEL_HERE',
	);

	return $custom_fields;
}, 10, 3 );

// If you need to format your value before it's displayed add this:
add_filter( 'et_builder_dynamic_content_meta_value', function ( $meta_value, $meta_key, $post_id ) {
	if ( 'META_KEY_HERE' === $meta_key ) {
		// Format your value here ...
	}

	return $meta_value;
}, 10, 3 );

Thanks!