Logstash: how to concatenate static field with dynamic field?

I'm trying to prepend a static variable to identify certain hosts

    mutate {
        add_field => { "[host][hostname]" => "UK__[app][host]" }
    }

I need to preprend "UK__" to all fields of app.host field. But the above gives me outcome as
host.hostname: UK_[app][host] literally and is not replacing it with the value. how to concatenate static field with dynamic field to create another field?

Ignore me.. found out a way ! putting curly brackets around it

mutate {
        add_field => { "[host][hostname]" => "UK__%{[app][host]}" }
    }