Add custom fields for users to the search in the wp-admin

search_user_custom_fields.php

<?php 
/**
 * Add custom fields for users to the search in the wp-admin
 */
function codepress_search_user_custom_fields( $user_query ) {

	// Fill in the custom fields you want to add to the search
	$custom_fields = array(
		'my_custom_field_1',
		'my_custom_field_2'
	);

	if ( ! is_admin() || empty( $_GET['s'] ) || ! empty( $user_query->query_vars['custom_field_search'] ) ) {
		return $user_query;
	}

	global $wpdb;

	$user_ids = $wpdb->get_col(
		$wpdb->prepare( "
		SELECT DISTINCT u1.user_id
		FROM {$wpdb->usermeta} u1
		INNER JOIN {$wpdb->usermeta} u2 ON u1.user_id = u2.user_id
		WHERE ( u1.meta_key IN ('" . implode( "','", $custom_fields ) . "') AND u1.meta_value LIKE '%%%s%%' )
		",
			esc_attr( $_GET['s'] )
		)
	);

	if ( $user_ids ) {
		$user_query->query_where .= " OR {$wpdb->users}.ID IN (" . implode( ',', $user_ids ) . ")";
	}

	$user_query->query_vars['custom_field_search'] = true;
}
add_action( 'pre_user_query', 'codepress_search_user_custom_fields' );