How to use a custom plugin?

Please help me!
How i can activate a custom plugun

<?php 

add_action('telegram_parse','telegramcustom_parse', 10, 2);

function telegramcustom_parse( $telegram_user_id, $text ) {
    $plugin_post_id = telegram_getid( $telegram_user_id );

    if ( !$plugin_post_id ) {
        return;
    }

    if ( $text == '/latestnews') {
	
	$args = array( 'numberposts' => '4' );
	$recent_posts = wp_get_recent_posts($args);
	$return_message = '';
	
	foreach( $recent_posts as $recent ){
		$return_message .= '['.$recent["post_title"].']('.get_permalink($recent["ID"]).')'.PHP_EOL;
	}
	wp_reset_query();
    
       telegram_sendmessage( $telegram_user_id,  $return_message);
    }

    return;
}

?>