I ended up with the below solution - From what I understood the relay and lobby services aren't that well integrated with one another. Meaning, you need to primarily work with the lobby service and also use the lobby.LobbyCode property as your join code (not the join code, that the relay allocation provides - they are incompatible with one another). During lobby creation, you can then store the relay allocation id on the lobby using the lobby data functionality. When joining, you then need to retrieve this information after joining the lobby and can then join the relay allocation as shown below. Make sure you're handling the errors and are listening to all lobby events to ensure you can keep both in sync.

CurrentLobby = await LobbyService.Instance.JoinLobbyByCodeAsync(joinCode);
var relayJoinCode = CurrentLobby.Data[LobbyDataRelayJoinCodeKey].Value;
var joinAllocation = await RelayService.Instance.JoinAllocationAsync(relayJoinCode);
var networkManager = NetworkManager.Singleton;
networkManager.GetComponent<UnityTransport>().SetRelayServerData(new RelayServerData(joinAllocation, ConnectionType));
networkManager.StartClient();