我只是试了一下，似乎效果不错：

function wpse_form_in_admin_bar() {
    global $wp_admin_bar;

    $wp_admin_bar->add_menu( array(
        \'id\' => \'wpse-form-in-admin-bar\',
        \'parent\' => \'top-secondary\',
        \'title\' => \'<form><input type="text" /><input type="submit" /> </form>\'
    ) );
}
add_action( \'admin_bar_menu\', \'wpse_form_in_admin_bar\' );

@tollmanz的建议可能会起作用，但为了避免您的文本超出菜单宽度，我建议

 function wpse_form_in_admin_bar() {
    global $wp_admin_bar;

    $wp_admin_bar->add_menu( array(
        \'id\' => \'wpse-form-in-admin-bar\',
        \'parent\' => \'top-secondary\',
        \'title\' => \'title_goes_here\',
\'meta\'   => array(
            \'target\'   => \'_self\',
            \'html\'     => \'<!-- Custom HTML that goes below the item --><form><input type="text" /><input type="submit" /> </form>\',
        ),

) 

);
}
add_action( \'admin_bar_menu\', \'wpse_form_in_admin_bar\' );