Public iperf Servers
There's a number of online speed test websites, but they are all intended to test residential connections of less than 100Mb. They are also non standard and reliy among other things on browser speeds.
Common usage for iperf would be:
iperf -c iperf.server.name
with iperf.server.name being the server that you are testing to.
A list of public iperf servers can be found here: https://iperf.fr/iperf-servers.php
Finally, a trick to using iperf without a server is to send iperf data to a router which has the target IP address null routed. This will only work with UDP as UDP packets are one directional. This will allow you to measure the max bandwidth of a connection, but without having to use a remote server of any kind. The client maxes out the connection to the router and the router then just drops the packets. It's a neat little hack to test bandwidth when you have access to a router but do not have a remote iperf server to test to.