We don't really support hidden Wi-Fi networks. The connection test actually performs a standard http request over port 80 to a pre-defined URL. The Internet Request action simply executes a standard request. IFTTT and Spotify Web Connect also connect to various https endpoint on their servers. So, as long as the firewall allows TCP traffic over port 80 and 443, it should work. Of course a DNS server (the one assigned by the DHCP server) at UDP port 53 must also be able to be reached, assuming you use domain names instead of raw IP addresses.