[WordPress][Press This] Sets a custom default post format and category for post drafts saved via PressThis.

press-this-custom-terms.php

<?php
/**
 * Plugin Name: Press This | Default Terms
 * Description: Sets a custom default post format and category for post drafts saved via Press This.
 * Version:     0.0.2
 * Author:      Caspar Hübinger
 * Author URI:  http://glueckpress.com/
 * License: GNU General Public License v2 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 */

defined( 'ABSPATH' ) or exit;

/**
 * Add category and post format to Press This post before saving
 *
 * See: https://github.com/WordPress/press-this/blob/master/class-wp-press-this-plugin.php#L159
 */
add_filter( 'press_this_save_post', function ( $post_data ) {

	// Post format must be supported by theme
	// Note: Idk why, but the post format in Press This itself when you save a
	// draft still says Standard. However, when you open the draft in the
	// regular editor, it shows the value from here; so this works, even if 
	// it looks like it doesn’t as long as you’re in Press This.
	$post_data['post_format'] = 'link';

	// Get category ID from slug
	$category = get_category_by_slug( 'example-category' );
	$category_id = absint( $category->term_id );

	if ( 0 < $category_id ) {
		$post_data['post_category'] = [ $category_id ];
	}

	return $post_data;
} );