possible answer to https://wordpress.org/support/topic/wp-to-sf-object-record-id/

save-sf-id-acf.php

<?php

add_action( 'object_sync_for_salesforce_push_success', 'save_sf_id_acf', 10, 5 );
function save_sf_id_acf( $op, $response, $synced_object, $object_id, $wordpress_id_field_name ) {
	// do things if the save succeeded
	// $op is what the plugin did - Create, Update, Upsert, Delete
	// $response is what was returned by the $salesforce class. sfapi->response
	// $synced_object is an array like this:
	/*
	$synced_object = array(
		'wordpress_object' => $object,
		'mapping_object' => $mapping_object,
		'queue_item' => false,
		'mapping' => $mapping,
	);
	*/
	
	// you might want to exit out if it's not a newly created record, like this:
	if ( 'Create' !== $op ) {
		return;
	}
    
	// $object_id is the salesforce object id
	// $wordpress_id_field_name is the name of the ID field in WordPress
	// so the WP post ID would be stored in $synced_object['wordpress_object'][ $wordpress_id_field_name ]
	$post_id = $synced_object['wordpress_object'][ $wordpress_id_field_name ];
	// you would need to figure out what $field_name should be using somehow
	update_field( $field_name, $object_id, $post_id ); // this is an ACF method
}