My firm regularly acts as the U.S. Agent for our international clients, so we have lots of experience with that. In general, the foreign firm in its own FURLS registration records must first designate the U.S. Agent. Then the prospective U.S. Agent receives a notification to either accept or decline that designation. To accept or decline, the prospective U.S. Agent does so using the corresponding selection from within either the foreign firm’s FURLS record, or from within the prospective U.S. Agent’s own FURLS record. My firm uses the latter approach as we have our own FURLS record separate from those of our clients. Here’s an FDA page that might help you with more specific details.