micropub-link.php

<?php

add_action( 'after_micropub', function( $input, $wp_args ) {
	if ( empty( $wp_args['ID'] ) || empty( $input['properties']['bookmark-of'][0] ) ) {
		// Micropub request did not involve a (bookmark) post.
		return;
	}

	set_post_format( $wp_args['ID'], 'link' );
}, 10, 2 );