Source

File: wp-includes/class-wp-customize-nav-menus.php

public function insert_auto_draft_post( $postarr ) {
		if ( ! isset( $postarr['post_type'] ) ) {
			return new WP_Error( 'unknown_post_type', __( 'Invalid post type.' ) );
		}
		if ( empty( $postarr['post_title'] ) ) {
			return new WP_Error( 'empty_title', __( 'Empty title.' ) );
		}
		if ( ! empty( $postarr['post_status'] ) ) {
			return new WP_Error( 'status_forbidden', __( 'Status is forbidden.' ) );
		}

		/*
		 * If the changeset is a draft, this will change to draft the next time the changeset
		 * is updated; otherwise, auto-draft will persist in autosave revisions, until save.
		 */
		$postarr['post_status'] = 'auto-draft';

		// Auto-drafts are allowed to have empty post_names, so it has to be explicitly set.
		if ( empty( $postarr['post_name'] ) ) {
			$postarr['post_name'] = sanitize_title( $postarr['post_title'] );
		}
		if ( ! isset( $postarr['meta_input'] ) ) {
			$postarr['meta_input'] = array();
		}
		$postarr['meta_input']['_customize_draft_post_name'] = $postarr['post_name'];
		$postarr['meta_input']['_customize_changeset_uuid']  = $this->manager->changeset_uuid();
		unset( $postarr['post_name'] );

		add_filter( 'wp_insert_post_empty_content', '__return_false', 1000 );
		$r = wp_insert_post( wp_slash( $postarr ), true );
		remove_filter( 'wp_insert_post_empty_content', '__return_false', 1000 );

		if ( is_wp_error( $r ) ) {
			return $r;
		} else {
			return get_post( $r );
		}
	}