ACF Recent Posts Widget before hook

acp_rwp_before.php

<?php 

remove_filter( 'acp_rwp_before', array( 'ACF_Helper', 'af_bf_content_filter' ) );

add_filter( 'acp_rwp_before', 'wp_doin_before_custom_text', 999, 3 );

/**
 * @hook acp_rwp_before
 */
function wp_doin_before_custom_text($before, $instance, $id) {
	// note that to make usage of all function arguments
	// you will need to provide the hook with priority and extra params argument
	
	// say we want to append custom author info to the widget with author CSS class
	// after each post before content
	if ( $instance['css'] === 'author' ) {
		$before .= '<h3>Posted by: ' . get_the_author() . '</h3><hr />';
	}

	return $before;
}
?>