Two verbs look alike here. 달다 'attach, hang on' covers fixing something to something else — GPS를 어떻게 달아서 (fitting a GPS unit), 오징어 이렇게 달고 (with dried squid pinned on) — and it is also the verb for leaving a comment online, which is what 용주가 와서 '내 친구 멋있다' 이렇게 달면 describes. The other 달다 is the adjective 'sweet': 이렇게 달거라 생각도 못 했어, 은은하게 달 때.