If you are using this on plugin you should use priority argument like 999 and it will work on plugin.

add_action( "customize_register""wpcb_theme_customize_register",999,1); function wpcb_theme_customize_register($wp_customize){ $wp_customize->get_setting('blogdescription')->transport='postMessage'; } 

If you are using this in a plugin you should use priority argument like 999 and it will work in the plugin.

add_action( "customize_register","wpcb_theme_customize_register",999,1);  function wpcb_theme_customize_register($wp_customize){ $wp_customize->get_setting('blogdescription')->transport='postMessage'; }