function sh_bbp_menu() {

    $forums_menu  = bbp_get_global_post_field( 'forum_menu',  'edit' );
    //var_dump($forums_menu); ?>

    <hr />

    <strong class="label"><?php esc_html_e( 'Add to Menu:', 'bbpress' ); ?></strong>
    <label class="screen-reader-text" for="forum_menu"><?php esc_html_e( 'Forums Menu', 'bbpress' ); ?></label>
    <input name="forum_menu" type="checkbox" id="forum_menu" value="<?php echo esc_attr( $forums_menu ); ?>" />
    <?php
}
add_action( 'bbp_forum_metabox', 'sh_bbp_menu' );