// add to functions.php or as standalone plug-in
// to add autorefresh see https://codex.wordpress.org/Theme_Customization_API#Part_3:_Configure_Live_Preview_.28Optional.29
function header_slogan_setting($wp_customize) {
$wp_customize->add_setting( ‘fl-logo-description’, array( ‘default’ => get_bloginfo(‘description’), ‘transport’ => ‘postMessage’ ) );
$wp_customize->add_control( ‘fl-logo-description’, array( ‘section’ => ‘fl-header-logo’, ‘label’ => ‘Site Slogan’, ‘type’ => ‘text’, ‘settings’ => ‘fl-logo-description’ ) );
}
add_action( ‘customize_register’, ‘header_slogan_setting’);