Enable searching by post ID in the WordPress Admin using SearchWP

functions.php

<?php

add_filter( 'searchwp_results', function( $results, $args ) {
	if ( ! is_admin() ) {
		return $results;
	}

	if ( count( $args['terms'] ) > 1 ) {
		return $results;
	}

	if ( is_numeric( $args['terms'][0] ) ) {
		$maybe_post = get_post( absint( $args['terms'][0] ) );
		if ( ! empty( $maybe_post ) ) {
			$results = array_merge(
				array( $maybe_post ),
				$results
			);
		}
	}

	return $results;
}, 10, 2 );